Update index_together fields in migrations

[tienne-B]

Migrations will fail when updating Django to 5.1 as the index_together model meta attribute has been removed, from both the models (these have already been changed), but also in the migrations. We should go through old migrations and update (or remove) instances that use index_together in a step.

If an index is already present as a unique_together, the step can be removed. Otherwise, an AddIndex operation should be added (or similar). We should not squash migrations.

https://docs.djangoproject.com/en/5.1/releases/4.2/#index-together-option-is-deprecated-in-favor-of-indexes