Limit CI concurrency (#524)

Saves some resources and should speed up PRs that push frequently.

https://docs.github.com/en/actions/using-jobs/using-concurrency

Author: bpkroth

.github/workflows/maven.yml

@@ -36,6 +36,10 @@ env:
   JAVA_VERSION: 21
   ERRORS_THRESHOLD: 0.01
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
+  cancel-in-progress: true
+
 jobs:
   compile-and-test:
     runs-on: ubuntu-latest