ci: Add mergify rule for conflict

Added mergify rules to post a message
when there is conflict on the PR.

ref: https://doc.mergify.io/examples.html#request-for-action

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

.mergify.yml

@@ -7,6 +7,15 @@ pull_request_rules:
       dismiss_reviews:
         approved: true
         changes_requested: false
+  - name: ask to resolve conflict
+    conditions:
+      - conflict
+    actions:
+      comment:
+        # yamllint disable-line rule:truthy
+        message: "This pull request now has conflicts with the target branch.
+        Could you please resolve conflicts and force push the corrected
+        changes? 🙏"
   - name: automatic merge
     conditions:
       - label!=DNM