[project] Code cleaning & migration to new API

- Task: https://www.odoo.com/web/#id=8847&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.kZxs1A0VXfYsHCv2
self.mapped('task_ids') and rec.task_ids are return only those record which has priority = '0' record not all records eg.(`project.task(22, 23, 25, 26)`).
that why use search method eg.(`project.task(24, 22, 23, 25, 26)`)