Automatically use createRoot for React@>=18 (#26279)
Also logs a message when defaulting, and a warning when using a prerelease build of `react-dom` (e.g. the React 18 alpha) which are not officially supported.
Note: I've done this in `webpack-config.ts` instead of the Next.js config, as we don't actually want you to be able to opt-out *without* downgrading back to React 17, and so it ought to be entirely removed from the config eventually.
