CI: upgrade to checkout@v4

Signed-off-by: Daniel Maslowski <info@orangecms.org>

Author: orangecms

.github/workflows/gh-pages.yml

@@ -12,7 +12,8 @@ jobs:
     concurrency:
       group: ${{ github.workflow }}-${{ github.ref }}
     steps:
-      - uses: actions/checkout@v2
+      - name: Checkout
+        uses: actions/checkout@v4
 
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v2