Unlock peer dependency on React to allow v17

* Fix to remove 15, which didn’t work anymore: this would normally be
  breaking (and we take deprecating older versions seriously), but as
  it didn’t work before, this change instead catches problems earlier
* Allow React 17 (and later) as a peer dependency: we can’t actually
  add code to test this yet because we’re waiting for our test
  frameworks to update too (see GH-498), but checking locally React 17
  seems supported

Related to: GH-498.