bump actions/checkout 2 -> 4

Author: avindra

.github/workflows/github-pages.yml

@@ -11,7 +11,7 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v2
         with: