apidiff: ignore adding entirely new API groups

Adding the new Lifecycle API group caused apidiff to complain because
this situation had not been encountered when setting up the initial set of
allow rules and wasn't handled.

We don't need a CHANGELOG.md entry for this because it is a normal Go API
change, so the entry gets removed.

Before:

    Incompatible changes:
    - ./informers.SharedInformerFactory.Lifecycle: added

After:

    Acceptable incompatible changes:
    - ./informers.SharedInformerFactory.Lifecycle: added
    - ./kubernetes.Interface.LifecycleV1alpha1: added