Update subscription's next billing date.

Next billing date has to be later than EOD today.
A date without a time (YYYY-MM-DD) is a date in the shop time zone.
That date gets the shop renewal billing time, or the time of the current next billing date when the shop has no renewal billing time.
If it is a paused subscription, it'll be reactivated.
If the subscription is in dunning, it'll be taken out of dunning.
UNDO_SKIP requires the skipEventId returned by Skip and ignores any supplied nextBillingDate.