Fix warning for django-filter -> “filter_class”

See: https://django-filter.readthedocs.io/en/stable/guide/migration.html#view-attributes-renamed-867

```
hypha/apply/api/v1/screening/tests/test_views.py::ScreeningStatusViewSetTests::test_staff_can_list_screening_statuses
hypha/apply/api/v1/screening/tests/test_views.py::ScreeningStatusViewSetTests::test_staff_can_view_screening_statuses_detail
  /Users/theskumar/work/hyphaapp/hypha/venv/lib/python3.9/site-packages/django_filters/rest_framework/backends.py:31: MigrationNotice: `ScreeningStatusViewSet.filter_class` attribute should be renamed `filterset_class`. See: https://django-filter.readthedocs.io/en/master/guide/migration.html See: https://django-filter.readthedocs.io/en/master/guide/migration.html
    filterset_class = self.get_filterset_class(view, queryset)
```