[FIX] hr_holidays: Include manager (parent_id) as valid responsible

On a time off request, if the 'Time Off Responsible' (leave_manager_id)
is not set, then the SUPERUSER_ID is taken as time off responsible for the
validation.

Before falling back on OdooBot, include the parent_id of the employee,
aka the manager, as a valid responsible.

closes odoo/odoo#39041

Task: 2072111
X-original-commit: 1e1b9b40
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>