Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/maven.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2.4.0
+    - uses: actions/checkout@v6.0.0
     - name: Set up JDK 1.8
       uses: actions/setup-java@v1.4.3
       with: