Upgrade checkout action to v4

Author: damccull

.github/workflows/publish-gh-pages.yml

@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout main
-        uses: actions/checkout@v3.0.0
+        uses: actions/checkout@v4
         with:
           submodules: true
       - name: Install zola