Fix DeprecationWarning creating naive datetime

[jacobg]

The following line of code:

python-ndb/google/cloud/ndb/model.py

Line 3912 in a441cea

return datetime.datetime.utcnow()

produces this error:

  /Users/j/.pyenv/versions/3.12.9/envs/n/lib/python3.12/site-packages/google/cloud/ndb/model.py:3900: 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).
    return datetime.datetime.utcnow()

There are some other lines of code that do the same thing.