Mark queue selector as deprecated

When routing rules system is used by default on GitLab.com, self-managed instances, and GDK, it's time to announce all parties that the queue selector is deprecated. The deprecation date can be on any minor release day. The actual code removal should be deferred until the next major release, maybe 16.0 17.0.

• Add a warning log when running Sidekiq cluster with --queue-selector and --negate options
• Announce at GitLab internally
  • #eng-week-in-review channel
  • #backend channel
  • Support engineer teams
• Add a migration document
• Draft a release note
• Verify routing rules documentation (weird, both queue selector and routing rules documents are not indexed at the moment)
• Add deprecation message in documents mentioning queue selector

Official deprecation issue

gitlab-org/gitlab#390787 (closed)