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).
Edited by Sean McGivern