Fix Django deprecation warnings

[benmezger]

PR fixes Django deprecation warnings.

This PR does not fix all deprecation warnings, as the rest of the deprecation warnings are coming out from external libraries we use, i.e:

devel/tests/test_reports.py: 10 warnings
  /Users/ben/workspace/archweb/.venv/lib/python3.13/site-packages/pgpdump/packet.py:272: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    self.creation_time = datetime.utcfromtimestamp(

Ideally, once we bump Django, we would want to upgrade these modules as well.

One that made my head spin a little was the following:

packages/tests/test_views.py::test_sitemap
  /Users/ben/workspace/archweb/.venv/lib/python3.13/site-packages/django/contrib/sitemaps/__init__.py:73: UnorderedObjectListWarning: Pagination may yield inconsistent results with an unordered object_list: <class 'main.models.Package'> QuerySet.
    return paginator.Paginator(self._items(), self.limit)

Though I tried ordering the Queryset, I am not fully sure where this is coming from.

[jelly]

Thanks!