Update redis-client to v0.20.0
What does this MR do and why?
This fixes a few bugs in the hiredis-client, which we don't use, and freezes the command prelude (https://github.com/redis-rb/redis-client/pull/173). The latter prevents an issue we found with the Peek adapter mutating the AUTH password (gitlab-com/gl-infra/scalability#2826 (closed)) after the upgrade to the redis v5 gem. That issue has since been fixed with !144388 (merged).
Edited by Stan Hu