Bump actions/upload-artifact from 4 to 6 (#71)

Author: dependabot[bot]

.github/workflows/build.yml

@@ -50,7 +50,7 @@ jobs:
       # or preview the website in a PR.
       - name: Upload HTML website as an artifact
         if: success()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: html-${{ github.sha }}
           path: _build/html