gh-38380: CI: Remove `centos-7`
    
<!-- ^ Please provide a concise and informative title. -->
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description below. -->
<!-- ^ For example, instead of "Fixes #12345" use "Introduce new method
to calculate 1 + 2". -->
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EOL 2024-06-30, CI now fails https://github.com/mkoeppe/sage/actions/run
s/9970528956/job/27549649531#step:11:66
```
#6 [with-system-packages 2/4] RUN yum install -y centos-release-scl
#6 0.267 Loaded plugins: fastestmirror, ovl
#6 0.357 Determining fastest mirrors
#6 0.867 Could not retrieve mirrorlist http://mirrorlist.centos.org/?rel
ease=7&arch=x86_[64](https://github.com/mkoeppe/sage/actions/runs/997052
8956/job/27549649531#step:11:65)&repo=os&infra=container error was
#6 0.867 14: curl#6 - "Could not resolve host: mirrorlist.centos.org;
Unknown error"
```

### 📝 Checklist

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- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: #38380
Reported by: Matthias Köppe
Reviewer(s): Frédéric Chapoton

Author: Release Manager

.devcontainer/portability-centos-7-devtoolset-gcc_11-minimal/devcontainer.json

@@ -47,7 +47,6 @@ on:
            "fedora-38",
            "fedora-39",
            "fedora-40",
-           "centos-7-devtoolset-gcc_11",
            "centos-stream-9-python3.9",
            "almalinux-8-python3.9",
            "almalinux-9-python3.11",

.devcontainer/portability-centos-7-devtoolset-gcc_11-minimal/portability-Dockerfile

@@ -1564,60 +1564,6 @@
 .. |codespace-fedora-40-maximal| image:: https://github.com/codespaces/badge.svg
    :target: https://codespaces.new/sagemath/sage?devcontainer_path=.devcontainer%2Fportability-fedora-40-maximal%2Fdevcontainer.json
 
-.. |image-centos-7-devtoolset-gcc_11-minimal-with-system-packages| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-system-packages/size?tag=dev&label=with-system-packages&color=%23696969
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-system-packages
-
-.. |image-centos-7-devtoolset-gcc_11-minimal-configured| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-configured/latest_tag?ignore=latest,dev,*-failed&label=configured&color=%23696969
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-configured
-
-.. |image-centos-7-devtoolset-gcc_11-minimal-with-targets-pre| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-targets-pre/latest_tag?ignore=latest,dev,*-failed&label=with-targets-pre&color=%23677895
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-targets-pre
-
-.. |image-centos-7-devtoolset-gcc_11-minimal-with-targets| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-targets/latest_tag?ignore=latest,dev,*-failed&label=with-targets&color=%236686c1
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-targets
-
-.. |image-centos-7-devtoolset-gcc_11-minimal-with-targets-optional| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-targets-optional/latest_tag?ignore=latest,dev,*-failed&label=with-targets-optional&color=%236495ed
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-minimal-with-targets-optional
-
-.. |codespace-centos-7-devtoolset-gcc_11-minimal| image:: https://github.com/codespaces/badge.svg
-   :target: https://codespaces.new/sagemath/sage?devcontainer_path=.devcontainer%2Fportability-centos-7-devtoolset-gcc_11-minimal%2Fdevcontainer.json
-
-.. |image-centos-7-devtoolset-gcc_11-standard-with-system-packages| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-system-packages/size?tag=dev&label=with-system-packages&color=%23696969
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-system-packages
-
-.. |image-centos-7-devtoolset-gcc_11-standard-configured| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-configured/latest_tag?ignore=latest,dev,*-failed&label=configured&color=%23696969
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-configured
-
-.. |image-centos-7-devtoolset-gcc_11-standard-with-targets-pre| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-targets-pre/latest_tag?ignore=latest,dev,*-failed&label=with-targets-pre&color=%235d8a4c
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-targets-pre
-
-.. |image-centos-7-devtoolset-gcc_11-standard-with-targets| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-targets/latest_tag?ignore=latest,dev,*-failed&label=with-targets&color=%2350ab2e
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-targets
-
-.. |image-centos-7-devtoolset-gcc_11-standard-with-targets-optional| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-targets-optional/latest_tag?ignore=latest,dev,*-failed&label=with-targets-optional&color=%2344cc11
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-standard-with-targets-optional
-
-.. |codespace-centos-7-devtoolset-gcc_11-standard| image:: https://github.com/codespaces/badge.svg
-   :target: https://codespaces.new/sagemath/sage?devcontainer_path=.devcontainer%2Fportability-centos-7-devtoolset-gcc_11-standard%2Fdevcontainer.json
-
-.. |image-centos-7-devtoolset-gcc_11-maximal-with-system-packages| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-system-packages/size?tag=dev&label=with-system-packages&color=%23696969
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-system-packages
-
-.. |image-centos-7-devtoolset-gcc_11-maximal-configured| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-configured/latest_tag?ignore=latest,dev,*-failed&label=configured&color=%23696969
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-configured
-
-.. |image-centos-7-devtoolset-gcc_11-maximal-with-targets-pre| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-targets-pre/latest_tag?ignore=latest,dev,*-failed&label=with-targets-pre&color=%238f6b8d
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-targets-pre
-
-.. |image-centos-7-devtoolset-gcc_11-maximal-with-targets| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-targets/latest_tag?ignore=latest,dev,*-failed&label=with-targets&color=%23b46eb2
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-targets
-
-.. |image-centos-7-devtoolset-gcc_11-maximal-with-targets-optional| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-targets-optional/latest_tag?ignore=latest,dev,*-failed&label=with-targets-optional&color=%23da70d6
-   :target: https://ghcr.io/sagemath/sage/sage-centos-7-devtoolset-gcc_11-maximal-with-targets-optional
-
-.. |codespace-centos-7-devtoolset-gcc_11-maximal| image:: https://github.com/codespaces/badge.svg
-   :target: https://codespaces.new/sagemath/sage?devcontainer_path=.devcontainer%2Fportability-centos-7-devtoolset-gcc_11-maximal%2Fdevcontainer.json
-
 .. |image-centos-stream-9-python3.9-minimal-with-system-packages| image:: https://ghcr-badge.egpl.dev/sagemath/sage/sage-centos-stream-9-python3.9-minimal-with-system-packages/size?tag=dev&label=with-system-packages&color=%23696969
    :target: https://ghcr.io/sagemath/sage/sage-centos-stream-9-python3.9-minimal-with-system-packages
 
@@ -2647,17 +2593,6 @@
    * -    ‑*maximal*
      - |image-fedora-40-maximal-with-system-packages| |image-fedora-40-maximal-with-targets-pre|
      -
-   * - **centos**-7-devtoolset-gcc_11 
-       
-          ‑*minimal*
-     - |image-centos-7-devtoolset-gcc_11-minimal-with-system-packages| |image-centos-7-devtoolset-gcc_11-minimal-with-targets-pre| |image-centos-7-devtoolset-gcc_11-minimal-with-targets| |image-centos-7-devtoolset-gcc_11-minimal-with-targets-optional|
-     - |codespace-centos-7-devtoolset-gcc_11-minimal|
-   * -    ‑*standard*
-     - |image-centos-7-devtoolset-gcc_11-standard-with-system-packages| |image-centos-7-devtoolset-gcc_11-standard-with-targets-pre| |image-centos-7-devtoolset-gcc_11-standard-with-targets| |image-centos-7-devtoolset-gcc_11-standard-with-targets-optional|
-     - |codespace-centos-7-devtoolset-gcc_11-standard|
-   * -    ‑*maximal*
-     - |image-centos-7-devtoolset-gcc_11-maximal-with-system-packages| |image-centos-7-devtoolset-gcc_11-maximal-with-targets-pre|
-     -
    * - **centos**-stream-9-python3.9 
 
           ‑*minimal*

.devcontainer/portability-centos-7-devtoolset-gcc_11-standard/devcontainer.json

@@ -864,7 +864,6 @@ setenv =
                      debian-{bullseye,bookworm,trixie,sid} \
                      linuxmint-{20.1,20.2,20.3,21,21.1,21.2,21.3} \
                      fedora-{30,31,32,33,34,35,36,37,38,39,40} \
-                     centos-7-devtoolset-gcc_11 \
                      centos-stream-9-python3.9 \
                      almalinux-{8-python3.9,9-python3.11} \
                      gentoo-python{3.10,3.11,3.12} \