Setup instructions for iOS do not conform to UIScene migration guide

[pamafe1976]

The instructions given in README.txt, in the section iOS Setup should be updated according to https://docs.flutter.dev/release/breaking-changes/uiscenedelegate

[gdurandrexel]

Can you please explain what needs to be updated?
Should UNUserNotificationCenter.current().delegate = self as? UNUserNotificationCenterDelegate be moved to didInitializeImplicitFlutterEngine?

[MaikuB]

Thanks for raising this. I have currently left this as is with a link the same doc that is mentioned as Flutter team themselves are providing the general guidance that they have also noted is subject to change. I was conscious that whatever is added could change again.

@gdurandrexel yep, more broadly speaking, my understanding of the docs is that's migrating all logic from application:didFinishLaunchingWithOptions to didInitializeImplicitFlutterEngine

I'm happy to review a PR though if someone community feels it is more beneficial to add more explicit info at this point in time and can help with this