[IMP] mrp: store wo hourly cost

This commit adds a new field to work orders to store the hourly cost of
its work center at the time of its completion. This is to avoid
inconsistent cost calculations when a work center's hourly cost is
changed after the WO is completed.

Part 2 (fix) of task: 2440068
Enterprise PR: odoo/enterprise#20169
Upgrade PR: odoo/upgrade#2727

Part-of: odoo/odoo#74951