build: upgrade github.com/redis/go-redis/v8 to github.com/redis/go-redis/v9

Related to #875 (closed). Note: I'm keeping the MaxRetries setting change mentioned in this issue for a separate MR. This one is the simplest MR possible to bump to v9.

Changelog for go-redis/v9: https://github.com/redis/go-redis/blob/master/CHANGELOG.md#v9-2023-01-30

Along the way we also had to:

• Upgrade github.com/eko/gocache/v2 to github.com/eko/gocache/v4: Only the latter is compatible with go-redis/v9. This also introduces the first Go generics-related change in the codebase (which is fine as we're running on Go 1.18+). Major changes since v2:

  • https://github.com/eko/gocache/releases/tag/v3.0.0
  • https://github.com/eko/gocache/releases/tag/lib%2Fv4.1.2

• Upgrade github.com/go-redis/redismock/v8 to github.com/go-redis/redismock/v9: Only the latter is compatible with go-redis/v9.

• Swapped github.com/globocom/go-redis-prometheus for github.com/trim21/go-redis-prometheus: The latter is a branch with go-redis/v9 compatibility (which the former lacks).