github: update to checkout@v4

Author: vapier

.github/workflows/gh-pages.yml

@@ -15,7 +15,7 @@ jobs:
       GIT_AUTHOR_EMAIL: github-actions[bot]@users.noreply.github.com
     steps:
     # Fetch the source and the latest gh-pages.
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - run: git fetch origin gh-pages:refs/remotes/origin/gh-pages
 
     # Account info from https://api.github.com/users/github-actions%5Bbot%5D