Update to redis-client v0.21.0

What does this MR do and why?

This removes the need to use the custom redis-client patch we introduced in !144960 (merged) (https://github.com/redis-rb/redis-client/pull/176) and improves the logging of Redis error messages (https://github.com/redis-rb/redis-client/pull/178) so we can better discern which Redis server is having issues.

Diff: https://my.diffend.io/gems/redis-client/0.20.0/0.21.0

Relates to #444275