Fix checkbox and radio hydration (#27401)

Fixes whatever part of https://github.com/facebook/react/issues/26876
and https://github.com/vercel/next.js/issues/49499 that
https://github.com/facebook/react/pull/27394 didn't fix, probably.

From manual tests I believe this behavior brings us back to parity with
latest stable release (18.2.0). It's awkward that we keep the user's
state even for controlled inputs, so the DOM is out of sync with React
state.

Previously the .defaultChecked assignment done in updateInput() was
changing the actual checkedness because the dirty flag wasn't getting
set, meaning that hydrating could change which radio button is checked,
even in the absence of user interaction! Now we go back to always
detaching again.