Update upload-pages-artifact action

Version 4 is available since August.

Author: n-osborne

.github/workflows/gh-pages.yml

@@ -28,7 +28,7 @@ jobs:
         uses: actions/configure-pages@v4
 
       - name: Upload artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v4
         with:
           path: _build/default/_doc/_html