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This migrates Flutter to use the `@main` attribute introduced in Swift 5.3. The `@UIApplicationMain` attribute is deprecated and will be removed in Swift 6. See: https://github.com/apple/swift-evolution/blob/main/proposals/0383-deprecate-uiapplicationmain-and-nsapplicationmain.md This change is split into two commits: 1.ad18797428- This updates the iOS app template and adds a migration to replace `@UIApplicationMain` uses with `@main`. 2.8ecbb2f29f- I ran `flutter run` on each Flutter iOS app in this repository to verify the app migrates and launches successfully. Part of https://github.com/flutter/flutter/issues/143044