Fix build warning

[Fishbowler]

Before:

% flutter build ios --simulator 

Building com.example.example for simulator (ios)...
Running pod install...                                             698ms

/Users/danc/git/maestro/demo_app/ios/Runner/AppDelegate.swift:13: warning: Flutter deprecation: Accessing rootViewController in `application:didFinishLaunchingWithOptions:`
[flutter-launch-rootvc].
        After the UISceneDelegate migration the `UIApplicationDelegate.window` and `UIWindow.rootViewController` properties will not be set in
        `application:didFinishLaunchingWithOptions:`. If you are relying on that in order to register platform channels at application launch use the `FlutterPluginRegistry` API
        instead. Other setup can be moved to a FlutterViewController subclass (ex: `awakeFromNib`).

Running Xcode build...                                                  
Xcode build done.                                           13.5s
✓ Built build/ios/iphonesimulator/Runner.app

Claude followed the advice and fixed the app, which appears to operate as before.