[FIX] account: correct communication when registering payment for multiple invoices

When a user register a payment for multiple invoices,
the communication field is populated using all invoices.
However the payment should use the communication individually for each invoice.
This is a behaviour closer to what is done in 74b6f84cc8 for v12.

opw 1954887

closes odoo/odoo#32143

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

addons/account/models/account_payment.py

@@ -194,11 +194,13 @@ class account_register_payments(models.TransientModel):
         '''
         amount = self._compute_payment_amount(invoices) if self.multi else self.amount
         payment_type = ('inbound' if amount > 0 else 'outbound') if self.multi else self.payment_type
+        communication = (' '.join([inv.reference or inv.number for inv in invoices])
+                         if self.multi else self.communication)
         return {
             'journal_id': self.journal_id.id,
             'payment_method_id': self.payment_method_id.id,
             'payment_date': self.payment_date,
-            'communication': self.communication,
+            'communication': communication,  # DO NOT FORWARD PORT TO V12 OR ABOVE
             'invoice_ids': [(6, 0, invoices.ids)],
             'payment_type': payment_type,
             'amount': abs(amount),