Currently, a product can only be part of one `pos_category`. This PR changes the relationship between the `product` and the `pos_category` from a `One2many` to a `Many2many`. closes odoo/odoo#122907 Task: 3138825 Related: odoo/enterprise#41791 Related: odoo/upgrade#4739 Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>