[FIX] l10n_it_edi: dates in FatturaPA import fix

Dates in FatturaPA are in ISO 8601 date format: `[-]CCYY-MM-DD[Z|(+|-)hh:mm]`
Data like `2021-01-01z` or `2021-02-05+01:00` did break the checks.

closes odoo/odoo#109712

Signed-off-by: Josse Colpaert <jco@odoo.com>