feat: remove optional private grafana

[merll]

This PR drops the per-team setting managedMonitoring.private, which was mainly intended to maintain backwards compatibility to the behavior before the multitenancy became the default, despite being potentially confusing.

Checklist

• The values-schema.yaml file and test/** fixtures have been updated to reflect code changes, if applicable.
• The OpenApi Schema from redkubes/otomi-api project is compatible with definitions from values-schema.yaml file, if applicable.
• Helm chart and helmfile changes are tested against upgrade scenario, if applicable.