Make EmailsOnPushWorker use Sidekiq mailers queue

EmailsOnPushWorker was using the default queue, which made it impossible
to prioritize MergeWorker and other key workers with a separate Sidekiq
process.

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.9.0 (unreleased)
   - Allow enabling wiki page events from Webhook management UI
+  - Make EmailsOnPushWorker use Sidekiq mailers queue
   - Fix wiki page events' webhook to point to the wiki repository
   - Allow forking projects with restricted visibility level
   - Improve note validation to prevent errors when creating invalid note via API

app/workers/emails_on_push_worker.rb

 class EmailsOnPushWorker
   include Sidekiq::Worker
+  sidekiq_options queue: :mailers
   attr_reader :email, :skip_premailer
   def perform(project_id, recipients, push_data, options = {})