Deprecate Model.Meta.index_together in favour of Model.Meta.indexes

[awais786]

The index_together option in Django model Meta classes has been deprecated and is now removed:

https://docs.djangoproject.com/en/5.1/releases/4.2/#features-deprecated-in-4-2

If this still exists in our codebase, we’ll encounter the following error during tests or migrations:

TypeError: ‘class Meta’ got invalid attribute(s): index_together

This isn’t just a minor cleanup — it affects database indexes and requires new migration.

The django-upgrade tool does not automatically detect or handle this issue, so it will require manual intervention.

[awais786]

My findings are in this PR openedx/edx-celeryutils#339

[awais786]

List of table with index_together

ExperimentData = ?
PersistentSubsectionGrade = ?
PersistentCourseGrade = ?
CourseSection = ?
StudentModuleHistoryExtended = ?

[UsamaSadiq]

Tested the potential effect of the index_together --> indexes change on prod db replica. We've verified that it won't be a blocker for us on the large tables as well. Further details are mentioned in the discovery issue specifically in the latest comment.

[awais786]

Multiple instances exists in openedx.

https://github.com/search?q=org%3Aopenedx%20index_together&type=code

Try to make small prs and merge one by one.

[UsamaSadiq]

The table with ~140M records had Time projection was (~15 seconds) but actual migration took only ~4 seconds so we are sure now that the db table is not being locked and it is taking ~3 seconds per index rename operation irrespective of the table size.