[React@18] Post-upgrade clean up

[Dosant]

As part of an upgrade to React@18 packages we introduced some workarounds

I am creating this issue to track the removal of those workarounds once we upgrade the runtime to React@18 and start transitioning to the Concurent Mode.

Tasks

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1. Clean up react@18 env [React@18] Env variable to use React@18 #193113
2. Clean react-dom/client introduced in [React@18] Upgrade @testing-library/react #198918 (comment)
3. Come up with a migration path from legacyRoot:true in testing-library introduced in [React@18] Upgrade @testing-library/react #198918 (comment) as part of the transition to Concurrent Mode
4. Clean console override to mute some of act warning
5. Remove testing-library/hooks dep, couldn't remove it in remove outstanding testing-library/react-hooks usages #205307 because EUI implicitly uses it

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)