Consistently return `None` from setter methods

[gadomski]

Throughout pystac, "setter" methods usually return None, e.g.:

• def set_owner(self, obj: Assets) -> None:
• def set_root(self, root: Catalog | None) -> None:
• def set_self_href(self, href: str | None) -> None:

However, there are two instances (that I can find) where setters return something:

• def set_collection(self, collection: Collection | None) -> Item:
• def set_owner(self, owner: STACObject | None) -> Link:

I think we should change those two methods to return None to make things consistent across the library. This can be part of a v2.0 since it is a breaking change.