Remove sidekiq rake tasks

https://gitlab.com/gitlab-org/gitlab/blob/master/lib/tasks/sidekiq.rake defines some tasks for starting and stopping Sidekiq. These don't appear to have had a meaningful change in five years (https://gitlab.com/gitlab-org/gitlab/commits/master/lib/tasks/sidekiq.rake), which is not necessarily a problem - there is nothing about them that screams that it is wrong to me.

However, I think that these are currently unused. We typically start process using runit: in development, that's via the GDK; for source installs, it's with https://gitlab.com/gitlab-org/gitlab/blob/master/lib/support/init.d/gitlab#L298; and for Omnibus, it's the default.

In gitlab-org/gitlab!22892 (merged) I wanted to add some Sidekiq-related Rake tasks, and went for a gitlab:sidekiq namespace, but I also found this existing sidekiq namespace.

Having these unused isn't the biggest problem in the world but if they are unused, we should probably remove them. I propose to:

1. Add a deprecation warning to all of these tasks, linking to this issue.
2. If this issue has no comments by 13.0, remove them.