Fix Mergify config

Convert deprecated `commit_message` attribute to
`commit_message_template`.  See https://docs.mergify.com/actions/merge/#defining-the-commit-message

.mergify.yml

@@ -9,7 +9,11 @@ pull_request_rules:
     queue:
       name: default
       method: squash
-      commit_message: title+body
+      commit_message_template: |
+        {{ title }} (#{{ number }})
+
+        {{ body }}
+
   name: Automatically merge pull requests
   conditions:
   - or: