Introduce workflow for stale PRs (#12813)

* Introduce stale workflow

* Exempt draft PRs

* Tune the action to our needs

1. Don't mark issues stale, only PRs.
2. Don't close anything automatically.
3. Keep the default Stale label.
4. Run in debug-only mode to start.

.github/workflows/stale.yml

@@ -0,0 +1,36 @@
+# This workflow warns of PRs that have had no activity for a specified amount of time.
+#
+# For more information, see https://github.com/actions/stale
+name: Mark stale pull requests
+
+on:
+  # Run every day at 00:00 UTC
+  schedule:
+  - cron: '0 0 * * *'
+  # Or run on demand
+  workflow_dispatch:
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+
+    steps:
+    - uses: actions/stale@v5
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+
+        days-before-pr-stale: 14
+        days-before-issue-stale: -1   # don't mark issues as stale
+        exempt-draft-pr: true         # don't mark draft PRs as stale
+
+        days-before-close: -1         # don't close stale PRs/issues
+
+        stale-pr-message: >
+          This PR has not had activity in the past 2 weeks, labeling it as stale.
+          If the PR is waiting for review, notify the dev@lucene.apache.org list.
+          Thank you for your contribution!
+
+        debug-only: true