Adding GitHub Actions workflow to close stale PRs (#15710)

.github/workflows/close_stale_prs.yml

@@ -0,0 +1,26 @@
+# Labels PRs without updates for 60 days as stale, then closes them 15 days later.
+name: Close stale PRs
+
+on:
+  schedule:
+    # Every day at 1:39 AM UTC. A random time to avoid GitHub Actions being overloaded by scheduled workflows.
+    - cron: '39 1 * * *'
+
+jobs:
+  close-stale-prs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          stale-pr-message: >
+            It seems that this pull request didn't really move for quite a while. Is this something you'd like to 
+            revisit any time soon or should we close? Pleaes comment if you'd like to pick it up and remove the 
+            "stale" label.
+          stale-pr-label: stale
+          # Not marking issues as stale.
+          days-before-issue-stale: -1
+          # Close the PR 15 days after marking it as stale.
+          days-before-close: 15
+          close-pr-message: >
+            Closing this pull request because it has been stale for very long. If you think this is still relevant, 
+            feel free to reopen it.