chore: update mergify for v29 (#9017)

Author: PaddyMc

.github/mergify.yml

@@ -203,6 +203,14 @@ pull_request_rules:
       backport:
         branches:
           - v28.x
+  - name: backport patches to v29.x branch
+    conditions:
+      - base=main
+      - label=A:backport/v29.x
+    actions:
+      backport:
+        branches:
+          - v29.x
   - name: automerge to the base branch with label automerge and branch protection
       passing
     conditions: []