[FIX] sale_purchase: disable PO stat button on SO
[FIX] sale_purchase: disable PO stat button on SO

Creating a service and stockable that create a PO on SO confirmation
(with different suppliers) will indeed work, but the status button
will only display 1 PO. The problem is that stock is not linking
the SO line with the PO line.
Thus, we prefer desactivated the button displaying the wrong value, as
it brings confusion to the user.
A proper bugfix should be done in master, to correctly link SO line with
PO line making the PO count correct.

opw-1905189

closes odoo/odoo#28410
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo