Update gitlab-experiment to 0.4.5

What does this MR do?

It updates the gem, where a few bug fixes have been implemented as we use it for the first time.

@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