`intl` 0.19.0 doesn't have any changes that break us, so we can expand our range of allowed versions for better ecosystem compatibility.

Fixes flutter/flutter#141779

Internal native helper code was still using `id` arguments, doing type checking, and handling `NSNull`, none of which is relevant now that the calling code has been converted to Pigeon and is already strongly typed as `NSString`.

The web tests currently all assume to be using the html renderer. `flutter test` previously erroneously was serving files for the HTML renderer by default (which is not the intended behavior). After this was fixed, the tests started running on the CanvasKit renderer and some of them broke. If we are relying on the html renderer, we should explicitly pass it when running `flutter test`

Should fix flutter/flutter#143177