ci: replace probot with actions workflow

Probot is basically discontinued, even though it's not 100% clear...

.github/workflows/stale.yml

@@ -0,0 +1,26 @@
+name: 'Close stale issues and PR'
+on:
+  schedule:
+    - cron: '00 5 * * *'
+  workflow_dispatch:
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v5
+        with:
+          days-before-stale: 20
+          days-before-close: 5
+          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.
+          stale-pr-message: >
+            This PR 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.
+          stale-issue-label: 'status: stale'
+          stale-pr-label: 'status: stale'
+          exempt-issue-labels: 'pinned,security,status: pinned,type: security'
+          exempt-pr-labels: 'pinned,security,status: pinned,type: security'