ci(Mergify): configuration update

Signed-off-by: Suraj Shetty <null>

.mergify.yml

@@ -1,4 +1,16 @@
 pull_request_rules:
+  - name: Auto-close PRs on stable branch
+    conditions:
+      - or:
+        - base=version-13
+        - base=version-12
+    actions:
+      close:
+      comment:
+          message: |
+            @{{author}}, thanks for the contribution, but we do not accept pull requests on a stable branch. Please raise PR on an appropriate hotfix branch. 
+            https://github.com/frappe/erpnext/wiki/Pull-Request-Checklist#which-branch
+
   - name: Automatic merge on CI success and review
     conditions:
       - status-success=Sider