docs: use warning admonition for legacyRoot

Author: MatanBobi

docs/react-testing-library/api.mdx

@@ -107,7 +107,9 @@ your components.
 
 ### `legacyRoot`
 
-**WARNING**: This option is only available when tests run with React 18 and earlier.
+:::warning
+This option is only available when tests run with React 18 and earlier.
+:::
 
 By default we'll render with support for concurrent features (i.e.
 [`ReactDOMClient.createRoot`](https://reactjs.org/docs/react-dom-client.html#createroot)).