dillon1000/react

Replace unbatchedUpdates with flushSync (#21776)

There's a weird quirk leftover from the old Stack (pre-Fiber)
implementation where the initial mount of a leagcy (ReactDOM.render)
root is flushed synchronously even inside `batchedUpdates`.

The original workaround for this was an internal method called
`unbatchedUpdates`. We've since added another API that works almost the
same way, `flushSync`.

The only difference is that `unbatchedUpdates` would not cause other
pending updates to flush too, only the newly mounted root. `flushSync`
flushes all pending sync work across all roots. This was to preserve
the exact behavior of the Stack implementation.

But since it's close enough, let's just use `flushSync`. It's unlikely
anyone's app accidentally relies on this subtle difference, and the
legacy API is deprecated in 18, anyway.