[Release] Modify mergify config for backporting to 8.16 branch (#4014)

Author: ycombinator

.mergify.yml

@@ -238,6 +238,19 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 8.16 branch
+    conditions:
+      - merged
+      - label~=^(backport-v8.16.0|backport-8.16)$
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.16"
+        labels:
+          - "backport"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
   - name: backport patches to 8.x branch
     conditions:
       - merged