feat: Change override date to datetime

When first building the Certificate Date Override feature, I set up the
CertificateDateOverride model to store the override dates as Dates
instead of DateTimes.

Turns out this is not how edX typically handles dates, and it’s causing
some minor headaches around needing to convert values. Also, using just
Dates causes timezone issues.

MICROBA-1488

openedx/core/djangoapps/programs/tests/test_tasks.py

@@ -587,7 +587,7 @@ class AwardCourseCertificatesTestCase(CredentialsApiConfigMixin, TestCase):
         assert call_args[1] == self.student.username
         assert call_args[2] == self.certificate
         assert call_args[3] == self.certificate.modified_date
-        assert call_args[4] == self.certificate.date_override.date.date()
+        assert call_args[4] == self.certificate.date_override.date
 
     def test_award_course_cert_not_called_if_disabled(self, mock_post_course_certificate):
         """