[1.8.x] Fixed #24725 -- Allowed renaming of target models in ManyToMany relations

This is a regression caused by introducing rendered migration states in
1aa3e09c and the _meta refactoring in fb48eb05.

Thanks to Danilo Bargen for reporting the issue and Marten Kenbeek and
Tim Graham for triaging the bug and providing the initial test case.

Backport of 63f9b633 from master