Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/test-java.yml

@@ -14,7 +14,7 @@ jobs:
     name: 'Build'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-java@v5
         with:
           distribution: "temurin"
@@ -29,7 +29,7 @@ jobs:
     name: 'Javadoc'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-java@v5
         with:
           distribution: "temurin"