Update peer dependencies

[redallen]

What

Upgrade peer dependencies across packages from:

  "peerDependencies": {
    "prop-types": "^15.6.1",
    "react": "^16.4.0",
    "react-dom": "^15.6.2 || ^16.4.0"
  }

to

  "peerDependencies": {
    "react": "^16.8.0",
    "react-dom": "^16.8.0"
  }

Why

Currently React 16.4 is listed as a peer dependency of react-core. However, our documentation uses React 16.8 since it uses React hooks.

Outside of reviews, we have no way of preventing contributors from using features outside of our peer dependencies like React hooks. A good solution it to just upgrade to upgrade our React peer dependency to React 16.8 in a breaking change release. While we're at it, we might as well remove prop-types (helps towards #3874 too) since Typescript is now a viable alternative and prop-types is an optional way to consume our library.

Supercedes #3435 / #3497

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[redallen]

Closed with breaking changes.