[FIX] l10n_fr_fec: use invoice/bill date when generating FEC

Install FR accounting and have a FR company setup with VAT
Create a bill with bill date different from accounting date
Export FEC under Accounting>Reporting>FEC

For the created bill the accounting date will be used, while it should
be taken first the bill date

opw-2680288

closes odoo/odoo#88281

X-original-commit: 2b1fc6a9
Signed-off-by: Quentin De Paoli <qdp@odoo.com>
Signed-off-by: Grazioso Andrea (agr) <agr@odoo.com>