Enable ValidatingAdmissionPolicy featureGate by default in sylva

Summary

For various validation policies, we're using kyverno to set-up API-level filtering using webhook matchConditions.

But this approach has various limitations, as explained in !3347 (merged), it would be much easier to use ValidatingAdmissionPolicy in many cases.

For that reason, we should consider enabling that featuregate in 1.2.x, in order to be able to use ValidatingAdmissionPolicy in 1.3 release.