[FIX] purchase: show confirmation date in locked PO

Steps to reproduce the bug:
- Go to purchase app > create a request for quotation
- click on confirm > Lock

Problem:
Date confirmation becomes invisible, and date order becomes visible

Solution:
As we have already confirmed the purchase order and the lock button only appears when the PO is confirmed,
it makes sense to leave the confirmation date visible

https://github.com/odoo/odoo/blob/13.0/addons/purchase/views/purchase_views.xml#L138-L139



opw-2612608

closes odoo/odoo#74980

X-original-commit: 4d21eafd
Signed-off-by: Djamel Touati <DjamelTouati@users.noreply.github.com>