Use squash merge method for mergify

Mergify is blocked with the error:

  Merge commits are not allowed on this repository.
  The repository configuration doesn't allow merge commits.
  The merge method configured in Mergify configuration must be
  allowed in the repository configuration settings.

.mergify.yml

@@ -8,7 +8,7 @@ pull_request_rules:
   - name: automatic merge
     actions:
       queue:
-        method: merge
+        method: squash
         name: default
     conditions:
       - label!=work-in-progress