accounting_l10n_fr: fix ValidDate computation for FEC
According to https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000027804775/ ValidDate is the date when the transaction was validated ( "16. La date de validation de l'écriture comptable"), this implementation was simply returning the modification date, which was not always correct, for example after causality state was recaculated.
This keeps the getModificationDate fallback for cases where FEC is
used with some non validated transactions, in order to keep compatibility