[Release] add-backport-next (#49991)

Co-authored-by: elasticmachine <infra-root+elasticmachine@elastic.co>

Author: elastic-vault-github-plugin-prod[bot]

.mergify.yml

@@ -391,3 +391,11 @@ pull_request_rules:
       backport:
         branches:
           - "9.3"
+  - name: backport patches to 9.4 branch
+    conditions:
+      - merged
+      - label=backport-9.4
+    actions:
+      backport:
+        branches:
+          - "9.4"