Mergify: add backport-9.0 (#6743)

Author: pierrehilbert

.mergify.yml

@@ -374,6 +374,19 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 9.0 branch
+    conditions:
+      - merged
+      - label=backport-9.0
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "9.0"
+        labels:
+          - "backport"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
 
   - name: backport patches to all active minor branches for the 8 major.
     conditions: