[FIX] purchase: move reference update to the right onchange
[FIX] purchase: move reference update to the right onchange

Move the code from commit ea1fc124 to the onchange on purchase_id.
The field vendor_bill_purchase_id is a technical one added in v12 to allow using
old vendor bills as template for new vendor bills.
However the logic on purchase_id was already in v11, so we match commit 14239733

opw 1962364

closes odoo/odoo#32661

Signed-off-by: Nans Lefebvre (len) <a573b540d2dde90ba6da3b9d6286c8ec60c10044@odoo.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo