Clean up dependabot config, add github-action group (#158408)

1. Delete bundler and docker, these directories no longer exist and we have no Docker or Gemfiles any more.
2. Update outdated reviewer list which was blocking any reviewers from being added. https://github.com/flutter/flutter/pull/157473#issuecomment-2433542624
3. Change github action update from daily to weekly
4. Add a `group` so all github action dependencies are updated in the same PR, instead of one-per-dependency. The PR will look like:
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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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See also https://github.com/flutter/packages/pull/8044

.github/dependabot.yml

@@ -3,38 +3,16 @@
 
 version: 2
 updates:
-  - package-ecosystem: "bundler"
-    directory: "/dev/ci/mac"
-    schedule:
-      interval: "weekly"
-    reviewers:
-      - "christopherfujino"
-      - "jmagman"
-      - "keyonghan"
-    labels:
-      - "team"
-      - "team-infra"
-      - "autosubmit"
-  - package-ecosystem: "docker"
-    directory: "/dev/ci/docker_linux"
-    schedule:
-      interval: "weekly"
-    reviewers:
-      - "christopherfujino"
-      - "jmagman"
-      - "keyonghan"
-    labels:
-      - "team"
-      - "team-infra"
-      - "autosubmit"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
+    groups:
+      all-github-actions:
+        patterns: [ "*" ]
     reviewers:
       - "christopherfujino"
       - "jmagman"
-      - "keyonghan"
     labels:
       - "team"
       - "team-infra"