Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/gradle.yml

@@ -11,7 +11,7 @@ jobs:
         java: [ '17', '21', '25' ]
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Java ${{ matrix.Java }}
       uses: actions/setup-java@v5
       with: