CI: Delete GitHub Actions caches once PR is closed (#6900)

## Problem

> Approaching total cache storage limit (9.25 GB of 10 GB Used)
> Least recently used caches will be automatically evicted to limit the
total cache storage to 10 GB. [Learn more about cache
usage.](https://docs.github.com/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy)

From https://github.com/neondatabase/neon/actions/caches

Some of these caches are from closed/merged PRs.

## Summary of changes
- Add a workflow that deletes caches for closed branches

.github/workflows/cleanup-caches-by-a-branch.yml

@@ -0,0 +1,32 @@
+# A workflow from
+# https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries
+
+name: cleanup caches by a branch
+on:
+  pull_request:
+    types:
+      - closed
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Cleanup
+        run: |
+          gh extension install actions/gh-actions-cache
+
+          echo "Fetching list of cache key"
+          cacheKeysForPR=$(gh actions-cache list -R $REPO -B $BRANCH -L 100 | cut -f 1 )
+
+          ## Setting this to not fail the workflow while deleting cache keys.
+          set +e
+          echo "Deleting caches..."
+          for cacheKey in $cacheKeysForPR
+          do
+              gh actions-cache delete $cacheKey -R $REPO -B $BRANCH --confirm
+          done
+          echo "Done"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO: ${{ github.repository }}
+          BRANCH: refs/pull/${{ github.event.pull_request.number }}/merge