Rename ClusterRing to ControllerRing

[timebertt]

What this PR does / why we need it:

This PR renames the ClusterRing resource to ControllerRing. See the linked issue for the motivation.

⚠️ Please consult the updated documentation and examples for how to adapt to this breaking change.

• The clusterring- string has been removed from the shard and drain labels.
• There is no migration path available. You need to manually clean up installed resources (CRD, RBAC, etc.), remove old labels, and install the new resources.
• The kind-related and namespace labels have been removed from the sharder metrics.

Which issue(s) this PR fixes:
Fixes #429

Special notes for your reviewer: