Update gitlab-experiment to 0.4.8

What does this MR do?

It updates the gem, where a few new concepts have been added to provide some control to the timing of enabling an experiment (this could be independent of rolling out a variant), and better exclusion capabilities. It also adds some nice rspec matchers that will be vetted before documenting and promoting.

A summary of the changes can be viewed at https://gitlab.com/gitlab-org/gitlab-experiment/-/compare/v0.4.5...v0.4.8

@gitlab-org/growth/engineers

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