[FIX] l10n_be_invoice_bba: fix mail template

The field reference_type has been renamed invoice_reference_type in v11.
The selection value 'bba' for that field has been renamed 'structured'.
We update the template to reflect these changes.

Discovered thanks to a forward port of that field up to v12.

closes odoo/odoo#30224

addons/l10n_be_invoice_bba/data/mail_template_data.xml

@@ -33,7 +33,7 @@ from ${object.company_id.name}.
 % endif
     <br/><br/>
 
-% if object.reference_type=='bba' and object.reference:
+% if object.invoice_reference_type=='structured' and object.reference:
     <p>Please use the following communication for your payment : <b>${object.reference}</b></p>
 % endif