Make sidekiq-cluster the default

This makes cluster for the Sidekiq chart default to true.

As described in #1989 (closed), and the documentation here, this is a breaking change:

1. When cluster is true, queues and negateQueues must be a string.
2. When cluster is false, queues and negateQueues must be an array.

We will continue to support the second option until 14.0: gitlab-com/gl-infra/scalability#337 (closed)

Closes #1989 (closed).