[FIX] website_sale: prevent users from modifying carts if a payment has been initiated

Bug: Users are able to modify their eCommerce cart, while a payment for
said cart is being processed. This means that the users can be charged
an amount that is different from the total listed on the associated
sales order. This also prevents the cart from being confirmed.

Steps to reproduce:
1. Install `website_sale` and `payment_test`.
2. Go to the /shop page, add a product to the cart and process checkout
3. On the payment page, select the `test` payment method and use the
   card number `4111 1111 1111 1111` with the expiry date 06/28 and the
   secret code `900`.
4. Open a product's page in another tab
5. Go back to the payment tab, click 'Pay Now' and quickly switch to the
   second tab and add the product to the cart
6. Check the cart's chatter: the confirmation failed because the cart's
   and transaction's amounts mismatch.

Fix: Force creating a new cart if the current one has started the
payment process

This commit is a backport of b261b85da0e8385a63efa4eeee883f92e3dcf58d

opw-3122102

closes odoo/odoo#114660

Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Co-authored-by: Antoine Vandevenne (anv) <anv@odoo.com>