`utcnow()` Deprecation Warning

`_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

```python
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).
```