[FIX] hr_holidays: Allow creating time off in the past

Purpose
=======

In the case a time off is created:
- In the past
- Using support documents

An invalid user error is raised in the interface, because the webclient
is sending values for the field 'supported_attachment_ids' (Example: [(6, 0, [])])
which is writing by inverse relationship on the field 'attachment_ids'.

On the other hand, it should be possible for an employee to add attachment
on the time off after it has begun.

closes odoo/odoo#104803

Taskid: 3032232
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>