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
Does this MR meet the acceptance criteria?
Conformity
Edited by Jeremy Jackson