React 17 support

[karudo]

React 17 released already, but peerDependencies contains "react": "^15.3.0 || 16", so there are npm warnings after React is updated

[adidahiya]

It's on the 4.0 roadmap. We first need to drop support for 15.x, which is still used in some of our code bases. ETA is roughly EOY.

[david-raine]

My project needs React 17 as it has multiple app root elements, so I updated ./node_modules/@blueprintjs/core.package.json directly with "react": "^15.3.0 || 16 || 17" and everything seems to be working well so far.

[dapetcu21]

It doesn't need to wait for 4.0. React 17's only breaking change is the moving of event handlers from document root to React root element, which is unlikely to cause problems. It's a stepping stone release for future compatibility.