Draft: Set default Sidekiq queues to default,mailers

What does this MR do?

gitlab-org/gitlab!59604 (merged) introduced routing rules logic on Rails app to route a job based on a set of rules. This mechanism allows us to scale our Sidekiq's job processing throughput on GitLab.com. However, we decided to keep the routing rules simple on self-managed instances, to route all jobs to the default queue and Sidekiq listens to only default and mailers queue.

This change only addresses the Sidekiq server to handle default and mailers queues only.

Related MRs:

• THIS MR IS BLOCKED BY gitlab-org/gitlab!97908 (merged) - Sidekiq client change in Rails app
• gitlab-org/omnibus-gitlab!6289 (closed) - Omnibus

Related issues

gitlab-com/gl-infra/scalability#1491 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated