Rename Covered Experience pubsubbeat to User Experience SLI

What

Rename pubsub topics to user-experience-sli.

This is not in use right now. I'm cleaning up and preparing the stage for gitlab-com/gl-infra/observability/team#4385 without leftovers.

Why

We are moving away from the term Covered Experience since it is an industry wide term used in the context of SLA, and will use User Experience SLI moving forward.

Related issue: gitlab-com/gl-infra/observability/team#4385