We've supported Redis 3 for a while, but we now ship and use Redis 5 now. We should first deprecate support for Redis 3: * [x] Update documentation (will be addressed in https://gitlab.com/gitlab-org/gitlab/-/issues/213749) * [x] Show warnings in `rake gitlab:check` that Redis 3 is outdated (will be addressed in https://gitlab.com/gitlab-org/gitlab/-/issues/213749) In GitLab 13.0, we should remove support for Redis 3: * [ ] `HighlightCache`: https://gitlab.com/gitlab-org/gitlab/blob/8e88429c280f30b92a78d2c54d0d6da16d615c56/lib/gitlab/diff/highlight_cache.rb#L120-129 * [x] `RepositorySetCache`: https://gitlab.com/gitlab-org/gitlab/blob/3f6faae312d5bdaebc8e007a99b8666e0a6bc3cd/lib/gitlab/set_cache.rb#L67-69