ci: add github action to clean up stale branches (rescript-lang#8227)

* ci: add github action to clean up stale branches

* switch to /maintenance

Author: jderochervlk

.github/workflows/branch_cleanup.yml

@@ -0,0 +1,21 @@
+name: Cleanup Stale Branches
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * *' # This schedule runs the workflow at midnight every day
+
+jobs:
+  cleanup-stale-branches:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Cleanup Stale Branches
+        uses: cbrgm/cleanup-stale-branches-action@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          repository: ${{ github.repository }}
+          # ignore-branches: ""
+          ignored-prefixes: "maintenance/"
+          last-commit-age-days: 90
+          dry-run: true
+          rate-limit: true