Support Django 5.0 db_default attribute.

A DB default is not considered backward incompatible, as the default value is kept on DB-level.
See https://docs.djangoproject.com/en/dev/releases/5.0/#database-computed-default-values

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 5.x.y
 
+Feature:
+- Support Django 5.0 `db_default` attribute (issue #275)
+
 Bug:
 - Don't detect 'IS NOT NULL' as backward incompatible changes (issue #263)