Sphinx: reverting upload-pages back to v4

Author: knutfrode

.github/workflows/sphinx-rtd.yml

@@ -62,7 +62,7 @@ jobs:
         uses: actions/configure-pages@v4
 
       - name: Upload artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v4
         with:
           name: docs
           path: ./docs/build/html