mergify: support backport label for 9.0 branch (#4816)

Author: v1v

.mergify.yml

@@ -26,6 +26,9 @@ pull_request_rules:
           To fixup this pull request, you need to add the backport labels for the needed
           branches, such as:
           * `backport-/d./d` is the label to automatically backport to the `/d./d` branch. `/d` is the digit
+          * `backport-active-all` is the label that automatically backports to all active branches.
+          * `backport-active-8` is the label that automatically backports to all active minor branches for the 8 major.
+          * `backport-active-9` is the label that automatically backports to all active minor branches for the 9 major.
           **NOTE**: `backport-skip` has been added to this pull request.
       label:
         add:
@@ -365,3 +368,16 @@ pull_request_rules:
         branches:
           - "8.18"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 9.0 branch
+    conditions:
+      - merged
+      - label=backport-9.0
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        labels:
+          - "backport"
+        branches:
+          - "9.0"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"