Upgrade to Sidekiq 6

We're currently running Sidekiq 5.2.

Sidekiq 6 was released a while ago: https://github.com/mperham/sidekiq/blob/master/Changes.md#60

At first glance, these breaking changes would not affect us (GitLab.com), because we already run sidekiq trough Sidekiq cluster. Though they might still affect some self-managed installs that run Sidekiq directly (gitlab-com/gl-infra/scalability#240 (closed) & gitlab-com/gl-infra/scalability#337 (closed)).

Sidekiq 5 is still getting security updates, it seems, so there is no urgency yet.

But there are some performance improvements we could benefit from, specifically in 6.0.1.

Furthermore, our own sidekiq-reliable-fetch main-line only supports Sidekiq 6, so it would be easier to support that if we were running the latest version.