Update actions/checkout action to v4

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - name: Git checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           submodules: recursive # fetch Hugo themes
           fetch-depth: 0