fix: replace timezone.utc with datetime.timezone.utc

[mschoettle]

timezone.utc is deprecated and therefore the following RemovedInDjango50Warning is emitted during our tests:

ERROR    easyaudit.signals.model_signals:model_signals.py:125 easy audit had a pre-save exception.
Traceback (most recent call last):
  File "<snip>/.venv/lib/python3.10/site-packages/easyaudit/signals/model_signals.py", line 72, in pre_save
    delta = model_delta(old_model, instance)
  File "<snip>/.venv/lib/python3.10/site-packages/easyaudit/utils.py", line 57, in model_delta
    new_value = get_field_value(new_model, field)
  File "<snip>/.venv/lib/python3.10/site-packages/easyaudit/utils.py", line 28, in get_field_value
    value = timezone.make_naive(value, timezone=timezone.utc)
  File "<snip>/.venv/lib/python3.10/site-packages/django/utils/timezone.py", line 49, in __getattr__
    warnings.warn(
django.utils.deprecation.RemovedInDjango50Warning: The django.utils.timezone.utc alias is deprecated. Please update your code to use datetime.timezone.utc instead.

This PR replaces the use of timezone.utc with datetime.timezone.utc.