[Release] add-backport-next (#36339) (#36346)

(cherry picked from commit 9baefee)

Co-authored-by: Elastic Machine <[email protected]>

.mergify.yml

@@ -263,3 +263,16 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 8.10 branch
+    conditions:
+      - merged
+      - label=backport-v8.10.0
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.10"
+        labels:
+          - "backport"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"