push_notifications.dart

import 'package:firebase_messaging/firebase_messaging.dart';

class PushNotificationsManager {

  PushNotificationsManager._();

  factory PushNotificationsManager() => _instance;

  static final PushNotificationsManager _instance = PushNotificationsManager._();

  final FirebaseMessaging _firebaseMessaging = FirebaseMessaging();
  bool _initialized = false;

  Future<void> init() async {
    if (!_initialized) {
      // For iOS request permission first.
      _firebaseMessaging.requestNotificationPermissions();
      _firebaseMessaging.configure();

      // For testing purposes print the Firebase Messaging token
      String token = await _firebaseMessaging.getToken();
      print("FirebaseMessaging token: $token");
      
      _initialized = true;
    }
  }
}

Some replacements are needed:

final FirebaseMessaging _firebaseMessaging = FirebaseMessaging.instance;

and

      _firebaseMessaging.requestPermission();
      // _firebaseMessaging.configure(); -> configure() has been removed

See: https://firebase.flutter.dev/docs/migration/#messaging