[Release] add-backport-next (#3875) (#3882)

Co-authored-by: Pierre HILBERT <pierre.hilbert@elastic.co>
(cherry picked from commit b5db8f5)

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

.mergify.yml

@@ -280,3 +280,16 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 8.12 branch
+    conditions:
+      - merged
+      - label=backport-v8.12.0
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.12"
+        labels:
+          - "backport"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"