Deprecate `COLLECTION_PER_CLASS` inheritance

[GromNaN]

Feature Request

Q	A
New Feature	no
RFC	yes
BC Break	yes

Summary

The the COLLECTION_PER_CLASS inheritance means there is 1 collection per Document class. Which is not different from how all the Document classes works.

Getting the example from #2726

#[ODM\InheritanceType('COLLECTION_PER_CLASS')]
#[ODM\DiscriminatorMap([3 => Tag::class, 4 => Category::class])]
class Section {}

class Tag extends Section {}
class Category extends Section {}

If you run a query on the Section repository, the section collection is queried. There is no way to get a Tag or Category object in this way.

If you run a query on the Tag repository, the tag collection is queried. There is no way to get a Category object in this way. The discriminator field is not necessary.

This feature has an issue as we cannot guarantee that 2 documents doesn't share the same _id, as they are in distinct collections. #2725 (comment)

This feature has almost no documentation: https://www.doctrine-project.org/projects/doctrine-mongodb-odm/en/2.9/reference/inheritance-mapping.html#collection-per-class-inheritance

[GromNaN]

I'm thinking about it now, maybe the use case is for polymorphic relationships.

Edit: that's not confirmed by the introduction commit 7613b91, there is no relationships in the examples and tests.