[FIX] sale: use correct decorator on prepare method
[FIX] sale: use correct decorator on prepare method

In `_prepare_procurement_group`, self.name is accessed.
The method should then use api.multi instead of api.model
Can not backbport as may be problematic if override in old API

Closes #15510
ygol/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,ygol/odoo