reactivated stale.yaml as github action (#3577)

The old `stale.yaml` seems not to work anymore, so I set up the same
content in a new github workflow.

I think we should use it again to get rid of old issues.

.github/workflows/stale.yaml

@@ -0,0 +1,20 @@
+name: "Close stale issues and PRs"
+
+on:
+  schedule:
+    - cron: "30 1 * * 6" # every Saturday at 1:30
+
+permissions:
+  issues: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          stale-issue-message: "This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions."
+          days-before-issue-stale: 60
+          days-before-issue-close: 7
+          stale-issue-label: "wontfix"
+          exempt-issue-labels: "pinned,security,under investigation,pr welcome"