Remove deprecated usage of `admission.Validator` and `admission.Defaulter` in v2 code

[wjf3121]

Describe the current behavior

admission.Validator and admission.Defaulter are removed from controller-runtime v0.20: kubernetes-sigs/controller-runtime#2877

This prevents us from upgrading controller-runtime in our codebase when azure-service-operator/v2 is used.

Describe the improvement

Remove the deprecated usage of admission.Validator and admission.Defaulter

Additional context

Add any other context about the suggested improvement.

[theunrepentantgeek]

Closely related to #4381, but we'll keep both open for tracking.