[FIX] account_edi_ubl_cii: prepend encoding to xml

Prepend the encoding `<?xml version="1.0" encoding="utf-8"?>` to every xml
in the account_edi_ubl_cii module.

The absence of this tag was causing a 'Document format not supported.' error on myguichet.lu.

closes odoo/odoo#107081

Signed-off-by: Nicolas Viseur (vin) <vin@odoo.com>