ci: update actions/checkout to v6

Author: kkrypt0nn

.github/workflows/deploy_website.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: recursive
           fetch-depth: 0