Use the default job queue to deliver emails

This is the default in Rails 6.1 for Action Mailer, just like it's the
default for Active Storage.

config/initializers/new_framework_defaults_6_1.rb

@@ -45,7 +45,7 @@ Rails.application.config.action_dispatch.ssl_default_redirect_status = 308
 Rails.application.config.active_record.legacy_connection_handling = false
 
 # Set the default queue name for the mail deliver job to the queue adapter default.
-# Rails.application.config.action_mailer.deliver_later_queue_name = nil
+Rails.application.config.action_mailer.deliver_later_queue_name = nil
 
 # Generate a `Link` header that gives a hint to modern browsers about
 # preloading assets when using `javascript_include_tag` and `stylesheet_link_tag`.