Use --queue-selector flag for Sidekiq

What does this MR do?

In gitlab-org/gitlab!46562 (merged), we made the Sidekiq queue selector available without the 'experimental' prefix. This MR:

1. Uses the new flag, which is just --queue-selector.
2. Also accepts a SIDEKIQ_QUEUE_SELECTOR env var. For now, we accept both this and SIDEKIQ_EXPERIMENTAL_QUEUE_SELECTOR. We'll support that until we do gitlab-com/gl-infra/scalability#646 (closed).

Related issues

gitlab-com/gl-infra/scalability#147 (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 (in gitlab-org/charts/gitlab!1631 (merged))