Group dependabot github-action update PRs, delete dead docker updates (flutter#8044)

Dependabot now supports [grouping dependencies](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups) into one PR instead of one per dependency.

Group github-action updates into one PR. Make github-actions update weekly instead of daily. Remove docker since it was removed in flutter#4822

This means a PR like flutter/packages#7813 will instead be grouped into one.
Here's an example of what that will look like: kubernetes-sigs/cluster-api#11347

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<img width="400" alt="Screenshot 2024-11-08 at 11 08 52�AM" src="https://github.com/user-attachments/assets/eebedf69-3fae-409b-9c88-c3afc8036c52">

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https://github.com/kubernetes-sigs/cluster-api/blob/e13abeaed2bd3d7b04abd9c9823e2f4185fa6599/.github/dependabot.yaml#L10-L12

I also updated the reviewers to stop the "One or more of the users or teams you specified is not a collaborator" [error](flutter/packages#7813 (comment)).

Part of flutter#148098

Author: jmagman

.github/dependabot.yml

@@ -3,23 +3,15 @@
 
 version: 2
 updates:
-  - package-ecosystem: "docker"
-    directory: "/.ci"
-    schedule:
-      interval: "weekly"
-    reviewers:
-      - "stuartmorgan"
-      - "ditman"
-    labels:
-      - "autosubmit"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
+    groups:
+      all-github-actions:
+        patterns: [ "*" ]
     reviewers:
-      - "keyonghan"
-      - "yusuf-goog"
-      - "ricardoamador"
+      - "stuartmorgan"
     labels:
       - "team"
       - "team: infra"