Summary:
Apps implementing `UISceneDelegate` no longer clear out images when backgrounded because `UIApplicationDidEnterBackgroundNotification` no longer gets fired.
## Changelog
[iOS] [Added] - UIScene support for RCTImageView
Pull Request resolved: https://github.com/facebook/react-native/pull/28141
Test Plan:
1. Create a new iOS app implementing `UISceneDelegate` or modify an existing one
2. Open a React view with some images
3. Switch to another app, or background the current app
4. Observe that `-[RCTImageView clearImageIfDetached]` gets called
Reviewed By: shergin
Differential Revision: D20009200
Pulled By: hramos
fbshipit-source-id: bdbf79d6cf56a295344c036b9225efec672fa780