[Release] add-backport-next (#3683)

Co-authored-by: elasticmachine <infra-root+elasticmachine@elastic.co>

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

.mergify.yml

@@ -233,3 +233,16 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 8.15 branch
+    conditions:
+      - merged
+      - label=backport-v8.15.0
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.15"
+        labels:
+          - "backport"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"