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
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`swift-format` alphabetizes imports. Alphabetize them in swift template files and integration tests. I found this as part of https://github.com/flutter/flutter/issues/41129 running `swift-import` on packages.