Eliminate use of django.utils.datetime_safe

[jennifer-richards]

Describe the issue

In at least ietf/meeting/models.py, the django.utils.datetime_safe library is used. The datetime_safe module is reportedly going to go away and as far as I have found is not documented outside of the source code.

It's unclear to me why this was used, but I think it should be straightforward to drop and replace with the standard datetime library.

Minor, but must be addressed before or when migrating to Django 4.

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