`utcnow()` Deprecation Warning

[pierre-phntm]

_now staticmethod, used by auto_now_add, is using datetime.datetime.utcnow(), which is deprecated.

Environment details

1. API: python-ndb
2. 2020 MacOSX; Sonama 14.5
3. Python 3.12.4
4. google-cloud-ndb version: 2.16.0

Steps to reproduce

1. Add auto_now_add to a DateTimeProperty model.

Code example

time_stamp_field = ndb.DateTimeProperty(tzinfo=datetime.timezone.utc, auto_now_add=True)

Stack trace

DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).