maint: update actions/upload-artifact action to latest version (#227)

Author: isabelizimm

.github/workflows/docs.yml

@@ -29,7 +29,7 @@ jobs:
         run: |
           make docs
       - name: save docs
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: docs-html
           path: docs/_site