Automatic merge from submit-queue

Move StorageClass to a storage group

We discussed the pros and cons in sig-api-machinery yesterday.  Choosing a particular group name means that clients (including our internal code) require less work and re-swizzling to handle promotions between versions.  Even if you choose a group you end up not liking, the amount of work remains the same as the incubator work case: you move the affected kind, resource, and storage.

This moves the `StorageClass` type to the `storage.k8s.io` group (named for consistency with authentication, authorization, rbac, and imagepolicy).  There are two commits, one for manaul changes and one for generated code.