[FIX] l10n_fr_pos_cert: Allow price modification

Currently, the module l10n_fr_pos_cert (France - VAT Anti-Fraud Certification):

Prevent price modification
Prevent from inserting negative lines while decreasing quantity
Hash links between orders
Print the hash onto the ticket

This commit allows the price modification of the products.
To be compliant with the law, we track any price modification on the ticket and on the product screen.

closes odoo/odoo#104429

Task-id: 2610576
Signed-off-by: Masereel Pierre <pim@odoo.com>