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.

changelogs/unreleased/add-warnings-to-sidekiq-rake-tasks.yml

+---
+title: Add deprecation warning to Rake tasks in sidekiq namespace
+merge_request:
+author:
+type: removed

lib/tasks/sidekiq.rake

 namespace :sidekiq do
-  desc "GitLab | Stop sidekiq"
+  def deprecation_warning!
+    warn <<~WARNING
+      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
+    WARNING
+  end
+
+  desc "[DEPRECATED] GitLab | Stop sidekiq"
   task :stop do
+    deprecation_warning!
+
     system(*%w(bin/background_jobs stop))
   end
 
-  desc "GitLab | Start sidekiq"
+  desc "[DEPRECATED] GitLab | Start sidekiq"
   task :start do
+    deprecation_warning!
+
     system(*%w(bin/background_jobs start))
   end
 
-  desc 'GitLab | Restart sidekiq'
+  desc '[DEPRECATED] GitLab | Restart sidekiq'
   task :restart do
+    deprecation_warning!
+
     system(*%w(bin/background_jobs restart))
   end
 
-  desc "GitLab | Start sidekiq with launchd on Mac OS X"
+  desc "[DEPRECATED] GitLab | Start sidekiq with launchd on Mac OS X"
   task :launchd do
+    deprecation_warning!
+
     system(*%w(bin/background_jobs start_no_deamonize))
   end
 end