Fix mailer jobs for deleted notifications erroring out (#16294)

Fixes an oversight in the Rails 6 migration

config/initializers/delivery_job.rb → config/initializers/mail_delivery_job.rb

-ActionMailer::DeliveryJob.class_eval do
+ActionMailer::MailDeliveryJob.class_eval do
   discard_on ActiveJob::DeserializationError
 end