ci: Add concurrency control to cancel outdated test runs

Add concurrency configuration to Maven Tests workflow to automatically
cancel previous runs when new commits are pushed to the same PR or branch.
This saves CI/CD resources and provides faster feedback.

Configuration:
- Groups runs by workflow name and PR number (or branch ref)
- Automatically cancels in-progress runs when new ones start

Author: claude

.github/workflows/maven-test.yml

@@ -8,6 +8,10 @@ on:
     branches:
       - main
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   test:
     runs-on: ubuntu-latest