Update gitlab-experiment to version 0.5.0 [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

It updates the gitlab-experiment gem. This is a seamless update and we'll circle back and remove some aspects from gitlab as we have availability. For now though, we've changed the API, but in such a way that would be invisible to existing experiments.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because dependency update.
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

I'm not very concerned about this update. The minor version bump is because we've done some things that could break backwards compatibility, but don't believe there's any uses of those things within gitlab.