[FIX] website_event_sale: use event_ticket_id to get event ticket

Expected Behaviour

When buying multiple tickets for an event on the website, we should
have the correct number of tickets in our cart

Obeserved Behaviour

When buying multiple tickets from an event B where the associated product
has already been associated to another event A , the maximum number of
tickets from the event A is used instead of the correct one (from B),
leading the client not to be able to buy as much ticket as he want in
case the limit of A is lower than the limit of B.

Reproducibility

This issue can be reproduced using the following steps :
1. Create an event A with a maximum number of 1 tickets and a product P
2. Buy and confirm 1 ticket
3. Copy the event A to an event B
4. Set the maximum number of tickets to 3 (keeping product P)
5. Go to the website page of event B and try to buy 3 tickets
6. On the checkout page you should have only 1 ticket for event B

Fix Description

The issue was coming from the fact we use the associated product to get
the event's ticket ID in the _cart_update mathod. This fix propose to use
the event_ticket_id value, already set in the sale_order context, to find
the correct ticket.

Related Issue(s)/PR

opw-2761303

closes odoo/odoo#88481

X-original-commit: db674e33
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Signed-off-by: Hendrickx Anthony (anhe) <anhe@odoo.com>