Add deprecation warning to tasks in sidekiq namespace

These tasks appear to be unused, but if someone sees this warning before 13.0, then they aren't, and we should ask them to comment on the issue 🙂

Example warning:

$ bundle exec rake sidekiq:stop
This task is deprecated and will be removed in 13.0 as it is thought to be unused.

If you are using this task, please comment on the below issue:
  https://gitlab.com/gitlab-org/gitlab/issues/196731

For #196731 (moved).