[FIX] purchase_stock: fallback on seller

- Create a product with routes MTO + Buy.
- Create a vendor pricelist with a minimum quantity:
  Min. Qty 1000 for 10 USD
- Create a SO, sell 1000 quantity
  => A PO is created automatically in draft.
- Go back to SO and update quantity to 1200.

The following message pops up: "There is no matching vendor price to
generate the purchase order for product..."

We select the supplier based on the procurement quantity, which is lower
than the minimum quantity of the supplier.

To avoid this situation, we fall back on any supplier like it was the
case in v12.

opw-2297001

closes odoo/odoo#54513

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>