[FIX] payment_sips: prevent clearing the session cookie

When buying a product on website shop, after the payment with SIPS, the
page is redirected to an Error message: "We are not able to find your
payment, but don't worry. You should receive an email confirming your
payment in a few minutes. If the payment hasn't been confirmed you can
contact us."

To reproduce the error:
1. In Payment Acquirers, enable Sips
2. Go on website shop
3. Add a product to the cart, Checkout
4. Pay with Sips
    - Visa card number: 4100000000000000
5. Back to Web-shop, if the payment has been successfully processed,
repeat steps 2 -> 4

Error: The message "Your payment has been successfully processed. Thank
you!" is not displayed. Instead, the message "We are not able [...] you
can contact us." is displayed.

This message is displayed when:
https://github.com/odoo/odoo/blob/5945806c151b13d9d4cc13aa0a6c96a6b1bbad5f/addons/payment/controllers/portal.py#L65-L69
i.e., when the transactions list is empty. Here is how to get the list:
https://github.com/odoo/odoo/blob/5945806c151b13d9d4cc13aa0a6c96a6b1bbad5f/addons/payment/controllers/portal.py#L38-L42
It uses the session of the request. The cookie `session_id` is used to
identify the current session. However, after the payment on SIPS, the
page is redirected to `/payment/sips/dpn` with a POST request. Since the
session cookie has the attribute `SameSite=Lax` and the HTTP request is
a POST, the cookie will be filtered out:
https://drive.google.com/file/d/1xfx3YWkfonO3nK-8Rew45uSoR4lkpjpY/view?usp=sharing


(Browser information: This cookie didn't specify a "SameSite" attribute
when it was stored and was defaulted to "SameSite=Lax," and was blocked
because the request was made from a different site and was not initiated
by a top-level navigation. The cookie had to have been set with
"SameSite=None" to enable cross-site usage)
As a result, the server creates a new one. This is the reason why the
transactions list is empty: the list is based on a new session.

Adding the attribute `save_session = False` to the route will prevent
the server from creating a new session cookie and add it in the POST
response.

OPW-2518377

closes odoo/odoo#72267

Signed-off-by: Antoine Vandevenne (anv) <AntoineVDV@users.noreply.github.com>