Remove ability to disable cluster from sidekiq values

What does this MR do?

Removes the cluster configurable, as we are now always using sidekiq cluster.

• Removes unnecessary queue support for old queue and negateQueue options
• Add deprecation notice for the cluster option

Related issues

Closes: #2747 (closed)

Test plan

• Using gitlab.sidekiq.cluster throws deprecation message
• Using .cluster in gitlab.sidekiq.pods... array throws deprecation message
• Uses all queues by default (you see the queue list using ps within the sidekiq pod)
• Setting queues works as expected (you can use ps once again to see the list)
• Setting negateQueues works as expected

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
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened