upgrade actions/checkout workflow to v4

Author: aidantomcy

.github/workflows/deploy.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout your repository using git
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Install, build, and upload your site
         uses: withastro/action@v0