Upgrade our Repo to React 17, tests, VR, examples, etc.

[ling1726]

[some partners are rolling out R17 now, others are planning]

[ling1726]

Enzyme does not officially support React 17 enzymejs/enzyme#2430

An unofficial adapter exists, but no guarantee all tests work https://www.npmjs.com/package/@wojtekmaj/enzyme-adapter-react-17

[ecraig12345]

Related: #15732, #16886

[ecraig12345]

After #22265, all the tests included in yarn test appear to be working with React 17 and @wojtekmaj/enzyme-adapter-react-17.

[ecraig12345]

Trying out React 17 again (draft): #22326

Probably everything works except a weird issue with unmounting northstar Popup in tests (not sure if it's a real issue too): #22326 (review)