Deprecate and remove support for Redis 3

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:

• Update documentation (will be addressed in #213749 (closed))
• Show warnings in rake gitlab:check that Redis 3 is outdated (will be addressed in #213749 (closed))

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
• RepositorySetCache: https://gitlab.com/gitlab-org/gitlab/blob/3f6faae312d5bdaebc8e007a99b8666e0a6bc3cd/lib/gitlab/set_cache.rb#L67-69