[Release] add-backport-next (#8694)

Co-authored-by: elasticmachine <[email protected]>

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

.mergify.yml

@@ -437,3 +437,11 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 9.1 branch
+    conditions:
+      - merged
+      - label=backport-9.1
+    actions:
+      backport:
+        branches:
+          - "9.1"