Register User Endpoint needs id for relations instead of documentId

## Bug report

### Required System information

Node.js version: 18.20.4
NPM version: 10.7.0
Strapi version: 5.3.0
Database: sqlite
Operating system: Mac OS / homebrew
Is your project Javascript or Typescript: JS

### Describe the bug

Using auth.register on Rest API we passed a documentId of a relation to connect ist while user is registered.
We get a sqLite Database Error. When we pass Id instead of documentId everything works fine.

### Steps to reproduce the behavior

1. Create Collection Type
2. Create an Entry in collection type
3. Create a ManyToOne Relation inside the collection type with user
4. Register a user via REST API and try to connect the entry (POST /api/auth/local/register)

```
{
...
  collectionTypePluralName : [ "CollectionTypesEntryDocumentId" ],
}
```

### Expected behavior

We expected that the documentId is needed for connecting relations on register user. Instead the Id is needed for now.

