[fleet] New API migration

- Task: https://www.odoo.com/web#id=10075&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.xFp6558VZvondGaA
```python
self.sum_cost = sum(cost.amount for cost in contract.cost_ids)
```