code: func (csrStrategy) PrepareForUpdate(ctx context.Context, obj, old runtime.Object) {
  	newCSR := obj.(*certificates.CertificateSigningRequest)
  	oldCSR := old.(*certificates.CertificateSigningRequest)
  
  	newCSR.Spec = oldCSR.Spec
  	newCSR.Status = oldCSR.Status
  }
docs: // PrepareForUpdate clears fields that are not allowed to be set by end users
  // on update. Certificate requests are immutable after creation except via subresources.