Reenable React "automatic" runtime

[eric-burel]

Describe the bug
"automatic" runtime of React 17 is creating issue with pragma/pragmaFrag in JSX (it is what handles <> syntax for React.Fragment as far as I understand and other global React)
To Reproduce

Remove runtime: "classic" in babel.config.js, it will raise the error on app build.

Expected behavior
We shouldn't need to use such hacks and be able to use runtime automatic.

It probably needs 3rd party packages like MDX to update. See mdx-js/mdx#1014

Additional context
Next 10 React 17