mergify: add 5.15 branch rule

Author: 0xc0170

.mergify.yml

@@ -180,6 +180,13 @@ pull_request_rules:
       label:
         remove: ['release version missing', 'Release review required']
 
+  - name: add release review required to release branches
+    conditions:
+      - base~=^mbed-os-5.15
+    actions:
+      label:
+        add: ['Release review required']
+
  # Clean-up if closed but not merged
   - name: remove release version and needs labels
     conditions: