Grow Redis conn pool size with Puma thread count

What does this MR do?

This supersedes !19778 (closed) to be a minimal but working quick-fix to not cap the Redis connection pool to a hard-coded number, but make it a function of the number of worker threads instead.

We intentionally left other things untouched here to be able to ship the smallest possible increment that adds value for 12.5 which is due soon.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

This will not have any impact on performance as long as we run Puma workers with a number of worker threads smaller than 5 + num_threads. So the immediate effect should be zero, but it gives us the option to scale of puma threads without causing contention on the Redis connection pool.

Closes #35559 (closed)