ci: Fix the concurrency group for db tests (no-changelog) (#7998)

The CI concurrency group was incorrectly named in
https://github.com/n8n-io/n8n/pull/7656.
This PR fixes that.

.github/workflows/ci-postgres-mysql.yml

@@ -9,7 +9,7 @@ on:
       - packages/cli/src/databases/migrations/**
 
 concurrency:
-  group: e2e-${{ github.event.pull_request.number || github.ref }}
+  group: db-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
 
 jobs: