Mark field_attach_* functions as deprecated

With #1969728: Implement Field API "field types" as TypedData Plugins and all the entity work in d8 field API attachers are obsolete: We do not provide a public API for working with some configurable field on an entity only, we only provide an API to work with the whole entity and fields.

E.g. you won't call field_attach_view() - instead you do use the entity render controller to view the whole entity. So let's make that explicit: You should not use field API attachers any more.

Right now, entity controllers partly still use field API attachers internally do their job, but that's subject to refactoring anyway such that all entity fields are handled the same. When we've done that, we can remove those deprecated functions later on.