[Release] add-backport-next (#5066)

Co-authored-by: elasticmachine <infra-root+elasticmachine@elastic.co>
(cherry picked from commit b46303b)

Author: 2 people

.mergify.yml

@@ -327,3 +327,11 @@ 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"