refactor: Renaming Labkit::CoveredExperience to Labkit::UserExperienceSli

What does this MR do and why?

Part of gitlab-com/gl-infra/observability/team#4347 (closed).

Depends on gitlab-org/ruby/gems/labkit-ruby!201 (merged).

The Labkit::CoveredExperience framework is being renamed to Labkit::UserExperienceSli. This MR updates the code using the framework to the recent version.

The log ingestion for the new log file is addressed in this MR gitlab-com/gl-infra/argocd/apps!234 (merged).

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.