Removes the cache and adds it to the gem (v0.4.12) [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Updates gitlab-experiment -- primarily to move the redis cache concept into the gem and out of gitlab.

There's also some updates to the cache interface in terms of additional attributes we might want to cache for experiments -- which will be used to enable rollout strategies. We also fixed a memoization issue where when not assigned into the experiment group (in the control), a context could get shifted into the experiment group when tracking an event or doing subsequent variant resolution.

https://gitlab.com/gitlab-org/gitlab-experiment/-/compare/v0.4.9...v0.4.12

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content