feat: add preserveUnknownFields=false marker

[aramase]

What this PR does / why we need it:

• Adds preserveUnknownFields=false marker for CRD generation. This is required for kubectl explain.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #214

Special notes for your reviewer:

➜ kubectl explain secretproviderclass                                                                                              
KIND:     SecretProviderClass
VERSION:  secrets-store.csi.x-k8s.io/v1alpha1

DESCRIPTION:
     SecretProviderClass is the Schema for the secretproviderclasses API

FIELDS:
   apiVersion   <string>
     APIVersion defines the versioned schema of this representation of an
     object. Servers should convert recognized schemas to the latest internal
     value, and may reject unrecognized values. More info:
     https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources

   kind <string>
     Kind is a string value representing the REST resource this object
     represents. Servers may infer this from the endpoint the client submits
     requests to. Cannot be updated. In CamelCase. More info:
     https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#types-kinds

   metadata     <Object>
     Standard object's metadata. More info:
     https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#metadata

   spec <Object>
     SecretProviderClassSpec defines the desired state of SecretProviderClass

   status       <Object>
     SecretProviderClassStatus defines the observed state of SecretProviderClass

/kind feature
/milestone v0.0.13

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aramase

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[aramase]

/retest

[ritazh]

/lgtm