Upgrade to Sidekiq 6.4

What does this MR do and why?

Upgrade contains a minor improvement to scheduled set processing: https://github.com/mperham/sidekiq/pull/5056

• https://github.com/mperham/sidekiq/blob/main/Changes.md
• https://my.diffend.io/gems/sidekiq/6.3.1/6.4.0
• https://github.com/mperham/connection_pool/blob/master/Changes.md
• https://my.diffend.io/gems/connection_pool/2.2.2/2.2.5

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.