Update react monorepo to ^16.12.0

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	References	Sourcegraph
react (source)	dependencies	minor	^16.12.0	homepage, source
react-test-renderer (source)	devDependencies	minor	^16.12.0	homepage, source

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Release Notes

facebook/react

v16.12.0

Compare Source

React DOM

• Fix passive effects (useEffect) not being fired in a multi-root app. (@​acdlite in #​17347)

React Is

• Fix lazy and memo types considered elements instead of components (@​bvaughn in #​17278)

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