Upgrade gitlab-experiment gem to 0.9.1

What does this MR do and why?

Update gitlab-experiment gem to use the latest version.

Diffs: https://gitlab.com/gitlab-org/ruby/gems/gitlab-experiment/-/compare/v0.8.0...v0.9.1?from_project_id=21250701&straight=false

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #428148 (closed)