refactor: Renaming Covered Experiences to User Experience SLIs

Hercules Merscherrequested to merge
rename-user-experience-slis into main

What

Renaming covered experience patterns to match with user experience SLIs changes in gitlab-org/gitlab!210524 (merged).

Why

We are moving away from the terminology Covered Experiences, used by the industry for SLAs, and using User Experience SLIs instead.

Links to relevant issues

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

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