Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
     - name: Configure git remote
       run: git config --add remote.origin.fetch '+refs/heads/*:refs/remotes/origin/*'
     - name: Fetch gh-pages branch