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.

Ran unit tests.