Update actions/checkout action to v6

Author: pschichbot

.github/workflows/publish.yaml

@@ -6,7 +6,7 @@ jobs:
   publish:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up JDK
         uses: actions/setup-java@v5
         with: