Bump Sidekiq 5.1.3 to 5.2.1
The most significant change in this version is that the default
concurrency has been lowered from 25 to 10 (https://github.com/mperham/sidekiq/issues/3892).
This doesn't affect omnibus-gitlab because the concurrency is controlled via a
setting that defaults to 25 anyway and is passed in via the
However, source installations (including the GDK) will have to either specify the concurrency in
sidekiq.yml or use the
Full list of changes: https://github.com/mperham/sidekiq/blob/master/Changes.md