[FIX] event_sale : compute attendees on cancel SO

Steps to reproduce:

- create a sale order with an event registration,
- on the event, the number of attendees is well computed
- cancel the sale order
- on the event, the number of attendees is still the same as before

Cause of the issue

There was no specific action for canceling a sale order linked
to an event registration

Solution

Add a action_cancel method which cancel also the sale order lines when
canceling the sale order

Also add a test to ensure the flow will not break in the future

opw-2576790

closes odoo/odoo#75102

X-original-commit: 12f96f85dc262c99b53bc843e41dd185eb5488e6
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Signed-off-by: guva-odoo <guva-odoo@users.noreply.github.com>