Move sidekiq-cluster to ee/bin

What does this MR do?

• sidekiq-cluster executable has been moved from bin/ to ee/bin/.
• References to sidekiq-cluster in documentations and tests has been updated accordingly.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Fixes #6565 (closed)