PMC-882 Make the necessary changes in the core according to the "Laravel" upgrade guide (From v5.6.30 to v5.7)

config/app.php

@@ -20,6 +20,7 @@ return [
         Illuminate\Queue\QueueServiceProvider::class,
         Illuminate\Translation\TranslationServiceProvider::class,
         Laravel\Tinker\TinkerServiceProvider::class,
+        Illuminate\Notifications\NotificationServiceProvider::class,
     ],
 
     'aliases' => [