Update Rails application to use redis-session instance

This will involve a new class Gitlab::Redis::Sessions inheriting from https://gitlab.com/gitlab-org/gitlab/blob/f1b74329658238a0014af6dd714e1b588ad7fa15/lib/gitlab/redis/wrapper.rb. It is probably similar to the TraceChunks class, falling back to the SharedState.

Migration will require care as we should not log everyone out at once and require re-auth (rude, and may cause problematic load side-effects), so we'll need to consider a multi-stage feature-flag approach (https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/development/redis/new_redis_instance.md#step-2-support-writing-to-and-reading-from-the-new-instance and step 3 & 4 after that) if there isn't some other way to identify (from the session id?) which store to use. The migration period may need to be quite long if we need to wait for inactive sessions to timeout before we consider the old instance unused.

Consideration should also be given to migrating miscellaneous session-related data as well, not just the literal session data (the data related to the key prefix constants in lib/gitlab/redis/shared_state.rb for example, although there may be others too).