Rename prometheus metrics namespace from 'baas' to 'k8up'

[ccremer]

Summary

• Renames metrics namespace from baas to k8up. It was meant to be included in the v2.0.0 release but was overseen.

NOTE: This is a breaking change for all K8up alert rules, as metrics like baas_backup_restic_last_errors will become k8up_backup_restic_last_errors. Adapt your alert rules if necessary.

Checklist

• Keep pull requests small so they can be easily reviewed.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking,
  as they show up in the changelog
• Update the documentation.