🧹 Change deprecrated Mergify rules (hedyorg#5738)

@rix0rrr pointed out to my attention that Mergify was complaining about some rules we were using incorrectly. The documentation says that they're now part of `queue_rules`.

**How to test**

We'll have to check this run in the following days.

.mergify.yml

@@ -4,6 +4,10 @@ queue_rules:
   - name: default_queue
     conditions:
         - "check-success=build"
+    merge_method: squash
+    commit_message_template: |-
+      {{ title }} (#{{number}})
+      {{ body }}
 
 pull_request_rules:
   - name: Automatic squash merge on approval
@@ -16,10 +20,4 @@ pull_request_rules:
     actions:
       comment:
         message: Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to [allow changes to be pushed to your fork](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork)).
-      queue:
-        name: default_queue
-        method: squash
-        commit_message_template: |-
-          {{ title }} (#{{number}})
-          {{ body }}