[Release] add-backport-next (#45060) (#45075)

(cherry picked from commit 0dd36b9)

Co-authored-by: elastic-vault-github-plugin-prod[bot] <150874479+elastic-vault-github-plugin-prod[bot]@users.noreply.github.com>
Co-authored-by: elasticmachine <infra-root+elasticmachine@elastic.co>

Author: 3 people

.mergify.yml

@@ -428,3 +428,11 @@ 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"