Fixed #36878 -- Unified data type for *_together options in ModelState.

Ever since the beginning of Django's migration framework, there's been a
bit of an inconsistency on how index_together and unique_together values
have been stored on the ModelState[^1].

It's only really obvious, when looking at the current code for
`from_model()`[^2] and the `rename_field()` state alteration code[^3].

The problem in the autodetector's detection of the `*_together` options
as raised in the ticket, reinforces the inconsistency[^4]: the old value
is being normalized to a set of tuples, whereas the new value is taken
as-is.

Why this hasn't been caught before, is likely to the fact, that we
never really look at a `to_state` that comes from migration operations
in the autodetector. Instead, in both usages in Django[^5], [^6] the
`to_state` is a `ProjectState.from_apps()`. And that state is
consistently using sets of tuples and not lists of lists.

[^1]: https://github.com/django/django/commit/67dcea711e92025d0e8676b869b7ef15dbc6db73#diff-5dd147e9e978e645313dd99eab3a7bab1f1cb0a53e256843adb68aeed71e61dcR85-R87
[^2]: https://github.com/django/django/blob/b1ffa9a9d78b0c2c5ad6ed5a1d84e380d5cfd010/django/db/migrations/state.py#L842
[^3]: https://github.com/django/django/blob/b1ffa9a9d78b0c2c5ad6ed5a1d84e380d5cfd010/django/db/migrations/state.py#L340-L345
[^4]: https://github.com/django/django/blob/b1ffa9a9d78b0c2c5ad6ed5a1d84e380d5cfd010/django/db/migrations/autodetector.py#L1757-L1771
[^5]: https://github.com/django/django/blob/2351c1b12cc9cf82d642f769c774bc3ea0cc4006/django/core/management/commands/makemigrations.py#L215-L219
[^6]: https://github.com/django/django/blob/2351c1b12cc9cf82d642f769c774bc3ea0cc4006/django/core/management/commands/migrate.py#L329-L332

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