Update Mergify configuration. (backport #7388) (#7389)

Per https://docs.mergify.com/actions/merge/#commit-message, the
commit_message option is deprecated and will be removed in 2022.
Replace it with the template suggested here:

https://docs.mergify.com/actions/queue/

(cherry picked from commit 02d456b8b8)

.github/mergify.yml

@@ -13,4 +13,7 @@ pull_request_rules:
       queue:
         method: squash
         name: default
-        commit_message: title+body
+        commit_message_template: |
+          {{ title }} (#{{ number }})
+
+          {{ body }}