gh-36697: CI Linux incremental: Set max_parallel = 8, reduce standard-sitepackages platforms
    
To reduce the impact of the PRs that run CI Linux incremental on the GH
Actions queue, in particular after a release (for example:
https://github.com/sagemath/sage/actions/runs/6827962772)

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URL: #36697
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Tobias Diez

Author: Release Manager

.github/workflows/ci-linux-incremental.yml

@@ -98,6 +98,7 @@ jobs:
           ["standard",
            "minimal"]
       docker_push_repository: ghcr.io/${{ github.repository }}/
+      max_parallel: 8
 
   site:
     needs: [changed_files]
@@ -120,16 +121,10 @@ jobs:
       docker_targets: "with-targets"
       targets: "${{needs.changed_files.outputs.uninstall_targets}} ${{needs.changed_files.outputs.build_targets}} build doc-html ptest"
       # Only test systems with a usable system python (>= 3.9)
+      # with only a small number of test failures as of 10.2.rc0
       tox_system_factors: >-
-        ["ubuntu-jammy",
-         "ubuntu-mantic",
-         "debian-bullseye",
-         "debian-bookworm",
-         "fedora-33",
-         "fedora-38",
-         "gentoo-python3.11",
-         "archlinux",
-         "debian-bullseye-i386"]
+        ["gentoo-python3.11",
+         "archlinux"]
       tox_packages_factors: >-
           ["standard-sitepackages"]
       docker_push_repository: ghcr.io/${{ github.repository }}/