Grouping GitHub Action updates in dependabot

In practice, all action updates are merged together.

So dependabot can group everything in one PR.

Config change required:

```diff
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "monthly"
+    groups:
+      github-actions:
+        applies-to: "version-updates"
```

## Acceptance criteria

- [ ] group created in all repos