Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/gradle.yml

@@ -14,7 +14,7 @@ jobs:
       contents: read
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up JDK 17
         uses: actions/setup-java@v5
         with: