Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/pkgdown.yaml

@@ -12,7 +12,7 @@ jobs:
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - uses: r-lib/actions/setup-r@v1