Consider removing admissionregistration.k8s.io/v1alpha1 runtime config

[gman0]

Commit https://github.com/kcp-dev/kcp/commit/c136c4ab01e6fd3570806b55cfd0c88e7707173e added a admissionregistration.k8s.io/v1alpha1 runtime config:

https://github.com/kcp-dev/kcp/blob/4bd4122483ff418a58315389b4f872dcaec0fe7c/pkg/server/options/options.go#L133-L136

This was added specifically to be able to serve ValidatingAdmissionPolicy when it was first introduced in k8s 1.26, as a part of admissionregistration.k8s.io/v1alpha1. Nowadays, VAP is in GA, and its v1alpha1 version is removed via APILifecycle in k8s v1.32, and its code will be completely removed in 1.33.

Consider removing the runtime config if it's not needed anymore. Also keep in mind that there is MutatingAdmissionPolicy, also currently in admissionregistration.k8s.io/v1alpha1 (our tracker https://github.com/kcp-dev/kcp/issues/3291), so depending on where and when the feature graduates next may influence the decision to remove or not.