Actions: downgrade actions/stale to v4 due to critical bug

The actions/stale GitHub Actions v5 has an issue where updates by the
GitHub Actions bot are recognized as updates. This causes the bot to not
work as intended as the Stale label will be removed.

This issue has already been reported at:
actions/stale#795

Let's downgrade the actions/stale package to v4 until the bug has been
fixed upstream.

.github/workflows/stale.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/stale@v5
+    - uses: actions/stale@v4
       with:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         days-before-stale: -1