inconsistent versioning of React and related packages

[telamonian]

Description

Currently, our handling of React versioning is a little... weird. Here's how we're currently versioning react, react-dom, and the related @types packages in core:

"react": "~16.12.0",
"react-dom": "~16.12.0",
"@types/react": "~16.9.16",   
"@types/react-dom": "^16.9.4",

Though guidance is a little thin, so far as I can tell the best practice would be for the minor version of all these packages to match. There's no newer versions of the @types packages yet, so I propose that we roll back react/react-dom to 16.9.x. Also, we should probably use a ~ for the version range of react-dom, instead of ^