[Release] add-backport-next (#4380)

Co-authored-by: elasticmachine <infra-root+elasticmachine@elastic.co>
(cherry picked from commit 5e1f441)

.mergify.yml

@@ -311,3 +311,16 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 8.18 branch
+    conditions:
+      - merged
+      - label=backport-8.18
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.18"
+        labels:
+          - "backport"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"