[FIX] mrp: determine order of mrp.production
[FIX] mrp: determine order of mrp.production

The order of production was first based on "priority DESC" and then
"date_planned ASC".

So for production with the same priority/date_planned the order is not
deterministic.

This commit make the order deterministic as it seems preferred.

opw-784243
closes #21644

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo