[FIX] purchase_requisition: Use correct UoM field

uom_po_id is the 'Purchase Unit of Measure' and is required so should be used
here.
This way, the onchange matches the behaviour of purchase module or the reset of
the purchase_requisition code

closes odoo/odoo#29717

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

addons/purchase_requisition/models/purchase_requisition.py

@@ -265,7 +265,7 @@ class PurchaseRequisitionLine(models.Model):
     @api.onchange('product_id')
     def _onchange_product_id(self):
         if self.product_id:
-            self.product_uom_id = self.product_id.uom_id
+            self.product_uom_id = self.product_id.uom_po_id
             self.product_qty = 1.0
         if not self.schedule_date:
             self.schedule_date = self.requisition_id.schedule_date