Without that field, GKE 1.21 refuses to create the CRD
with an error message that conversionReviewVersions is mandatory.
conversionReviewVersions is a required field when creating apiextensions.k8s.io/v1 custom resource definitions.
Webhooks are required to support at least one ConversionReview version understood by the current and previous API server.
See https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/_print/#webhook-request-and-response
Mário Uhrík
Yusuke Kuoka
Moto Ishizawa