Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/gradle.yml

@@ -8,7 +8,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Set up JDK
       uses: actions/setup-java@v5
       with: