Check for iOS 13 for Apple Signin

AppleSigninCheck.dart

// IMPORTANT: I'll need to fix this when we go past iOS 13. 
// Assume the following logic is inside a State<StatefulWidget>
supportsAppleSignIn = false;

@override
void initState() {
  super.initState();

  // initialize the AuthAgent
  this.authAgent = AuthAgent();

  // check if the device is iOS 13 or higher
  if (Platform.isIOS) {
    DeviceInfoPlugin().iosInfo.then((IosDeviceInfo iosDeviceInfo) {
      String version = iosDeviceInfo.systemVersion;
      print("iOS version is: $version");
      if (version.startsWith('13')) {
        setState(() {
          this.supportsAppleSignIn = true;
        });
      }
    });
  }
}