Fix mail delivery job deprecation (rails/rails#34591)

Author: Matthijsy

config/application.rb

@@ -22,6 +22,7 @@ module Amber
   class Application < Rails::Application
     # Initialize configuration defaults for originally generated Rails version.
     config.load_defaults 5.0
+    Rails.application.config.action_mailer.delivery_job = "ActionMailer::MailDeliveryJob" # TODO remove when defaults are set to 6.0
 
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration can go into files in config/initializers