Add dependabot github-actions group (flutter/engine#56468)

`.github/dependabot.yml` configures dependabot to update the `uses` dependencies needed to run github actions.  

For example, dependabot opened https://github.com/flutter/engine/pull/56191 and updated all the `.github/workflows/` yaml files to point to the latest version of `actions/checkout`

Add a dependabot `group` so all github action dependencies are updated in the same PR, instead of one-per-dependency.  For example, if there were an update for `actions/checkout` and `actions/setup-python`, dependabot will send out one PR updating both instead of one PR per dependency.
0d965cb949/.github/workflows/third_party_scan.yml (L23-L28)

New PRs from dependenabot 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 and https://github.com/flutter/flutter/pull/158408

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/.github/dependabot.yml

@@ -11,10 +11,14 @@ updates:
       timezone: "America/Los_Angeles"
     labels:
       - "autosubmit"
+    groups:
+      all-github-actions:
+        patterns: [ "*" ]
     ignore:
       # ignore patch versions, just rely on minor in order to update fewer times
       - dependency-name: "github/codeql-action"
         update-types: ["version-update:semver-minor"]
+
   - package-ecosystem: "pub"
     directory: "/lib/web_ui"
     schedule: