Use `Runtime.max_threads` when sizing Redis pools

What does this MR do?

Part 2/2 of #35170 (closed): This MR here only covers Redis pool settings, not DB.

It is a very small follow up to the work in !20210 (merged) and !21990 (merged) and merely uses the APIs introduced in those changes to determine connection pool size in multi-threaded environments like Puma or Sidekiq.

It is a refactoring only and should not change the way we size pools per-se.

NOTE: The diff looks large because it depends on unmerged changes in !22385 (merged).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content