DateTime library uses deprecated datetime.utcnow()

[pekkaklarck]

datetime.datetime.utcnow() has been deprecated in Python 3.12 and datetime.datetime.now(datetime.UTC) should be used instead. That datetime.UTC is new in Python 3.11, so we need to use either version or feature detection to avoid using it with earlier Python versions.

[pekkaklarck]

We cannot simply use datetime.datetime.now(datetime.UTC) because that returns a timezone aware datetime object that cannot be used together with non-timezone aware (i.e. naive) objects that we otherwise use with DateTime. Using datetime.datetime.now(datetime.UTC).replace(tzinfo=None) avoids that problem by removing timezone information and making the object naive.

DateTime also supporting aware objects and having better timezone support in general would be nice, but that requires quite a lot of changes. Can be considered later, but absolutely not in RF 7.0.