Make last_seen attribute a timezone aware datetime in UTC (#16348)

The last_seen attribute was a datetime in the local timezone but with
no tzinfo (i.e., a "naive" datetime.) When state changes occurred it
would be printed incorrectly in homeassistant.log because
homeassistant.util.dt.as_local assumes any datetime without tzinfo is
UTC. Also most, if not all, datetime attributes are timezone aware in
UTC. So use homeassistant.util.dt.as_utc (which assumes a naive
datetime is local) to convert last_seen to a timezone aware datetime
in UTC.