Fix MUI 4.12 deprecation warnings

Closes marmelab#6451

Author: fzaninotto

package.json

@@ -3,7 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "dependencies": {
-    "@material-ui/core": "^4.11.2",
+    "@material-ui/core": "^4.12.1",
     "@material-ui/icons": "^4.11.2",
     "@nivo/bar": "^0.67.0",
     "@nivo/core": "^0.67.0",