GVK is hard-coded to wrong coordinates

[rhuss]

E.g. in

• client/pkg/kn/commands/service_get.go

  Lines 50 to 51 in bd7d57f

  	Group: "knative.dev",
  	Version: "v1alpha1",

• client/pkg/kn/commands/service_describe.go

  Lines 54 to 57 in a264f06

  	describeService.GetObjectKind().SetGroupVersionKind(schema.GroupVersionKind{
  	Group: "knative.dev",
  	Version: "v1alpha1",
  	Kind: "Service"})

the GroupVersionKind is hardcoded.

In fact, it should be obtained via from the client library and not hardcoded.