This reverts merge request !149690

Deprecated in https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/2760
in favor of the new `concurrency` option.

Changelog: removed

The new concurrency option is a fixed value that doesnt depend on the
queues length. It is means to be a simple replacement over the
min-concurrency and max-concurrency.
https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/2760

Changelog: added

- spec/services/**
- spec/sidkiq*/**
- spec/support/**

sidekiq-cluster handles process supervision for its child Sidekiq
processes, and terminates itself and all child processes if any child
Sidekiq process exits.

Previously, it always exited with a 0 status code (i.e. success), no
matter how the child process had terminated.

Now it exits with 1 if any child process had a non-zero exit code. This
allows a process supervisor one level up (like systemd) to detect
failures and restart.

Changelog: changed

The previous default of 50 was far too high. We don't use this on
GitLab.com and it's mostly inertia that has left it at this value.

Changelog: changed

These methods are not directly related to SidekiqCluster process
management. They are currently only used by sidekiq cluster,
but in the future they may be used in other places as well.

This prevents name clashes with library code that
is auto-loaded in workers from lib/