Add stale PR workflow (#1220)

.github/workflows/pr-stale.yml

@@ -0,0 +1,18 @@
+name: "Close stale PRs"
+on:
+  schedule:
+  - cron: "0 0 * * *" # end of every day
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/stale@28ca1036281a5e5922ead5184a1bbf96e5fc984e # v9.0.0
+      with:
+        ascending: true
+        operations-per-run: 100
+        days-before-stale: 15
+        days-before-close: 7
+        stale-pr-message: "This PR is more than 2 weeks old. Please remove the
+          stale label, update, or comment if this PR is still valid and
+          relevant, otherwise it will be closed in 7 days."