Moving React 18 work from 3.6 into the 3.7 branch

[hwillson]

We're getting closer to launching 3.6 and since R18 isn't final yet, we'll move our useSyncExternalStore work into #9394

[hwillson]

Now that React 18 has launched, we're going to hold on the launch of 3.6 until we get the useSyncExternalStore changes back in. This work is being tracked in #8419, so I'll close this off.