refactor: Switch to using course_overview start and end everywhere

In DE-1822, we believed we needed to switch to start_date and end_date.
It was determined this was not the case, so this updates the comment
to ensure future users use the correct fields (start and end) and
updates any pieces of code that may have used start_date or end_date.

openedx/core/djangoapps/content/course_overviews/models.py

@@ -76,12 +76,15 @@ class CourseOverview(TimeStampedModel):
     display_number_with_default = TextField()
     display_org_with_default = TextField()
 
-    # Start/end dates
-    # TODO Remove 'start' & 'end' in removing field in column renaming, DE-1822
     start = DateTimeField(null=True)
     end = DateTimeField(null=True)
+
+    # These are deprecated and unused, but cannot be dropped via simple migration due to the size of the downstream
+    # history table. See DENG-19 for details.
+    # Please use start and end above for these values.
     start_date = DateTimeField(null=True)
     end_date = DateTimeField(null=True)
+
     advertised_start = TextField(null=True)
     announcement = DateTimeField(null=True)
 
@@ -194,10 +197,7 @@ class CourseOverview(TimeStampedModel):
         course_overview.display_org_with_default = course.display_org_with_default
 
         course_overview.start = start
-        # Add writes to new fields 'start_date' & 'end_date'.
-        course_overview.start_date = start
         course_overview.end = end
-        course_overview.end_date = end
         course_overview.advertised_start = course.advertised_start
         course_overview.announcement = course.announcement
 

openedx/core/djangoapps/credit/services.py

@@ -103,7 +103,7 @@ class CreditService:
             course_overview = CourseOverview.get_from_id(course_key)
             result.update({
                 'course_name': course_overview.display_name,
-                'course_end_date': course_overview.end_date,
+                'course_end_date': course_overview.end,
             })
         return result