[Release] add-backport-next (#5589)

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

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

.mergify.yml

@@ -254,3 +254,11 @@ pull_request_rules:
       backport:
         branches:
           - "9.1"
+  - name: backport patches to 9.2 branch
+    conditions:
+      - merged
+      - label=backport-9.2
+    actions:
+      backport:
+        branches:
+          - "9.2"