datetime.utcnow() deprecation in Python 3.12

[neilsh]

datetime.utcnow() was deprecated in Python 3.12, although it's referenced in quite a few places in MongoEngine's docs and test code, e.g.:

Note: To default the field to the current datetime, use: DateTimeField(default=datetime.utcnow)

It seems like it would make sense to do one of the following:

• Update docs to e.g. DateTimeField(default=lambda: datetime.now(timezone.utc))
• Create a timezone-aware helper called something like aware_utcnow, with behavior compatible across Python versions

[SOORAJTS2001]

Hi @neilsh, I just made a PR for it #2900