Django 5.1 removes support for `Meta.index_together` in favor of just including these in `Meta.indexes`. Any metadata classes that still include this unconditionally will cause a crash when attempting to finalize the model classes during initialization. We already had a `supports_index_together` flag, so we just needed to add some conditionals. A later change will add one additional test that runs on all versions for verifying that an evolution which removes `index_together` and adds `indexes` continues to run correctly even on Django>=5.1. Testing Done: Ran unit tests.