Use a ConnectionPool for Rails.cache on Sidekiq servers

What does this MR do?

On Sidekiq server we'll use a connection pool to connect to the redis store used for Rails cache. But now we're sure we're not modifying the configuration used on the lazy create Redis connection inside the connection pools. I've create a PR on redis-activesupport too. @jacobvosmaer-gitlab make this easier updating the Gitlab::Redis class !6472 (merged)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22364 (closed)