Update actions/checkout action to v6

Author: pschichbot

.github/workflows/gradle.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     environment: ${{ (github.ref == 'refs/heads/master' || startsWith(github.ref, 'refs/tags/javacan-') ) && 'sonatype-central' || null }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - name: Set up JDK