Update ReactUpdates-test (#28061)

## Overview

These tests are important for `ReactDOM.render`, so instead of just
re-writing them to `createRoot` and losing coverage:
- Moved the `.render` tests to `ReactLegacyUpdates`
- Re-wrote the tests in `ReactUpdates` to use `createRoot`
- Remove `unstable_batchedUpdates` from `ReactUpdates`

In a future PR, when I flag `batchedUpdates` with a Noop, I can add the
gate to just the tests in `ReactLegacyUpdates`.

Author: rickhanlonii