Add stalebot. (#496)

.github/workflows/stale.yml

@@ -0,0 +1,31 @@
+---
+# Close Stale Issues: Warns and then closes issues and PRs that have had no
+# activity for a specified amount of time.
+# See: https://github.com/actions/stale
+name: "Stalebot"
+
+on:
+  schedule:
+    - cron: "4 4 * * *"
+  workflow_dispatch:
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@main
+        id: stale
+        with:
+          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."
+          close-issue-message: ""
+          days-before-stale: 30
+          days-before-close: 3
+          exempt-draft-pr: true
+          exempt-issue-labels: "pinned,security"
+          exempt-pr-labels: "pinned,security,dependencies"
+          operations-per-run: 120
+      - name: Print outputs
+        run: echo ${{ join(steps.stale.outputs.*, ',') }}