[FIX] hr_fleet: no duplicate contract renewal activity reminder

Step to reproduce:
- Have a fleet contract that need renewal

Current Behaviour:
- A new activity is created everytime the scheduler is called

Behaviour after PR:
- A new activity is only created if there is no current renewal activity present when the scheduler is called

opw-2713537

closes odoo/odoo#81671

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>