[iOS] Migrate @UIApplicationMain attribute to @main (#146707)

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

dev/benchmarks/complex_layout/ios/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,