Update actions/checkout action to v6 (#8)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout code
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
     - name: Configure git remote
       run: git config --add remote.origin.fetch '+refs/heads/*:refs/remotes/origin/*'
     - name: Fetch gh-pages branch