Github Action for Closing Stale PRs (#1299)

Few PRs are left open for a long time without any follow-up activity 
from the author of the PRs. 

This prevents other developers from taking up the issue and solving 
it. Therefore, the senior developers need to find such PRs manually 
and close them.

Let's automate the process of closing inactive PRs using GitHub 
Actions.

.github/workflows/stale.yml

@@ -0,0 +1,29 @@
+name: Close Stale Pull Requests
+
+on:
+  schedule:
+  # Runs every day at 00:00 UTC
+  - cron: '0 0 * * *'
+
+jobs:
+  stale:
+    runs-on: ubuntu-18.04
+
+    steps:
+    - uses: actions/[email protected]
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        days-before-stale: 30
+        days-before-close: 7
+        remove-stale-when-updated: 'True'
+        stale-pr-label: 'Stale'
+        stale-pr-message: |
+          Hi,
+
+          We are going to mark this PR as stale because it has been inactive for the past 30 days.
+
+          If no further activity occurs within the following 7 days, it will be automatically closed so that others can take up the issue.
+          If you are still working on this PR, please make a follow-up commit within 7 days and leave a comment to remove the stale label.
+
+          Do let us know if you are stuck so that we can help you!'
+