[C2] Remove user.registration_date

[ywarnier]

The user table still maintains a registration_date field which is a functional duplicate of user.created_at.

• Remove registration_date in the entity.
• add a migration to deal with the field from 1.11.x, verifying that an empty created_at will be filled by the registration_date value before deletion.
• verify (including in backup/restore code) that this doesn't cause issues with user imports, user exports, cronjobs or other places where registration_date might be used. In such cases, just replace by created_at.