ci: bump actions/checkout to v6 and upload-pages-artifact to v4

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/deploy-website.yml

@@ -18,7 +18,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install uv
         uses: astral-sh/setup-uv@v7
@@ -32,7 +32,7 @@ jobs:
         run: uv run python website/build.py
 
       - name: Upload artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v4
         with:
           path: website/output/