build: comment about auto close when stalled via with github action

As part of automatically closing issues and PRs 30 days after they got
labelled with `stalled`, these changes adds a GitHub Action workflow
posting a comment information about what will happen in 30 days upon
being labelled.

PR-URL: #34555
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>

.github/workflows/comment-stalled.yml

@@ -0,0 +1,20 @@
+name: Comment on issues and PRs when labelled stalled
+on:
+  issues:
+    types: [labeled]
+  pull_request_target:
+    types: [labeled]
+
+jobs:
+  staleComment:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Post comment
+        if: github.event.label.name == 'stalled'
+        env:
+            COMMENTS_URL: ${{ github.event.issue.comments_url || github.event.pull_request.comments_url }}
+        run: |
+          curl -X POST $COMMENTS_URL \
+          -H "Content-Type: application/json" \
+          -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" \
+          --data '{ "body": "This issue/PR was marked as stalled, it will be automatically closed in 30 days. If it should remain open, please leave a comment explaining why it should remain open." }'