chore: stop Mergify from making new PRs to validate existing PRs it's about to merge (#34946)

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*By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license*

Author: rix0rrr

.mergify.yml

@@ -1,4 +1,7 @@
 # See https://doc.mergify.io
+merge_queue:
+  # Hopefully this gets Mergify to stop creating PRs just to test merges (it both messes with statistics and our automatic validations)
+  max_parallel_checks: 1
 queue_rules:
   - name: default-merge
     update_method: merge