Revert "Revert "This stage does the following: (#22692)""

This reverts commit 84de6bc6de
which reverts commit cebeab4348
which implements the first stage of the `values` column rename in
SiteConfiguration.  However, I included a small change:

This time, we set a default value on the new `site_values` column so
that the ORM will happily deserialize the JSONField without throwing a
JSONDecodeError.

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

@@ -146,6 +146,7 @@ class CertChangedReceiverTest(TestCase):
         mock_is_learner_issuance_enabled.return_value = True
 
         site_config = SiteConfigurationFactory.create(
+            site_values={'course_org_filter': ['edX']},
             values={'course_org_filter': ['edX']},
         )