Move cluster health feature into GitLab Core

This is an implementation issue to move our cluster health within the Monitor stage into Core

This should have a seperate MR to update our documentation as well.

we should probably close/verify #219992 (closed) once this issue gets implemented

Technical notes

• Related to the core version yml config/prometheus/cluster_metrics.yml, @splattael said: We should then move the EE version into the CE and update the code where the EE version was referenced 👍 (#223693 (comment 367479300))