chore(ci): limit job concurrency

see https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/

Author: lburgazzoli

.github/workflows/master-snapshot-release.yml

@@ -1,4 +1,7 @@
 name: Test & Release Snapshot to Maven Central
+concurrency:
+  group: ${{ github.ref }}-${{ github.workflow }}
+  cancel-in-progress: true
 on:
   push:
     branches: [ master ]

.github/workflows/pr.yml

@@ -1,4 +1,7 @@
 name: Verify Pull Request
+concurrency:
+  group: ${{ github.ref }}-${{ github.workflow }}
+  cancel-in-progress: true
 on:
   pull_request:
     branches: [ master ]