ios: Take automatic migration away from @UIApplicationMain

When building for iOS (`flutter run`), I got this output:

  ios/Runner/AppDelegate.swift uses the deprecated
    @UIApplicationMain attribute, updating.

And sure enough, this change was made, automatically. Sure; might as
well commit it. The following seems to be the explanation for it:
  https://github.com/apple/swift-evolution/blob/43b894b0c/proposals/0383-deprecate-uiapplicationmain-and-nsapplicationmain.md

Author: chrisbobbe

ios/Runner/AppDelegate.swift

@@ -1,7 +1,7 @@
 import UIKit
 import Flutter
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,