ci(migrations.yml): Limit concurrency in PRs for migrations workflows (#38897)

.github/workflows/migrations.yml

@@ -1,6 +1,12 @@
 name: migrations
 on: pull_request
 
+# Cancel in progress workflows on pull_requests.
+# https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-a-fallback-value
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   did-migration-change:
     name: check if any migration changes