Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/maven.yml

@@ -11,7 +11,7 @@ jobs:
         java-version: [ 8, 11, 15 ]
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
     - uses: actions/setup-java@v1
       with:
         java-version: ${{ matrix.java-version }}