dillon1000/react

Remove data-reactroot from server rendering and hydration heuristic (#20996)

This was used to implicitly hydrate if you call ReactDOM.render.

We've had a warning to explicitly use ReactDOM.hydrate(...) instead of
ReactDOM.render(...). We can now remove this from the generated markup.
(And avoid adding it to Fizz.)

This is a little strange to do now since we're trying hard to make the
root API work the same.

But if we kept it, we'd need to keep it in the generated output which adds
unnecessary bytes. It also risks people relying on it, in the Fizz world
where as this is an opportunity to create that clean state.

We could possibly only keep it in the old server rendering APIs but then
that creates an implicit dependency between which server API and which
client API that you use. Currently you can really mix and match either way.