Native many-to-many relationships

[bart-degreed]

One of the new features in EF Core 5 is native support for many-to-many relationships without explicitly mapping the join table.

This removes the need for our HasManyThroughAttribute, which we intend to remove in the next major version. The migration for existing APIs should be easy and we can't think of any reason to keep [HasManyThrough].

Our example for self-referencing many-to-many relationships is still possible but requires somewhat special mapping (spoiler here). Other than that, we need to update our EF Core inverse navigation resolver to also capture skip-nativations (this is what many-to-many are called under the covers in EF Core), so we can filter out existing join table entries from an incoming POST to-many request.

As part of this change, we should update documentation and examples accordingly.