ci: set up angular-automatic-lock-bot for the repo (#16975)

Author: josephperrott

.github/workflows/lock-closed.yml

@@ -0,0 +1,15 @@
+name: Lock issues that are closed and inactive
+
+on:
+  schedule:
+    # Run at the top of every hour
+    - cron: '0 * * * *'
+
+jobs:
+  lock_closed:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: angular/dev-infra/github-actions/lock-closed@14ebea5
+        with:
+          lock-bot-key: ${{ secrets.LOCK_BOT_PRIVATE_KEY }}
+          locks-per-execution: 250