AppDelegate.swift: overriding declaration requires an 'override' keyword

You have two application methods, the second one should be overridden as follows

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
  override func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
    // Store the completion handler.
    AWSS3TransferUtility.interceptApplication(application, handleEventsForBackgroundURLSession: identifier, completionHandler: completionHandler)
  }
}