[FIX] pos: do not mix create and validation date

The validation_date was using the creation_date value in the follwing scenario:
1. create a new order with a few lines (creation_date=t1)
2. reload the page, `init_from_JSON` uses validation_date=t1
3. validate the order at t2, ignored due to already stored value

creation_date must be kept for tracablity reasons (e.g. pos_blackbox_be)

Fixes https://www.odoo.com/groups/59/20542134