Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/publish.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
 
     - name: Login to GitHub Container Registry
       uses: docker/login-action@v1