macos: Take automatic migration away from @NSApplicationMain

This change was made automatically by `flutter run`,
along with the following warning:

  macos/Runner/AppDelegate.swift uses the deprecated
    @NSApplicationMain attribute, updating.

This change is the macOS counterpart of a previous commit
for iOS:
  zulip@5f111cb

macos/Runner/AppDelegate.swift

@@ -1,7 +1,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true