State of this notification, which can take one of the following
values:
ACTIVE: This outage notification is active. The event could be in
the past, present, or future. See start_time and end_time for
scheduling.
CANCELLED: The outage associated with this notification was
cancelled before the outage was due to start. Note that the
versions of this enum prefixed with "NS_" have been deprecated in
favor of the unprefixed values.