Updated validation logic for bank account fields

We've refined the description of the bank account validation logic to improve clarity and reduce ambiguity.

Overview:

• iban is preferred over bank_account_number if both are provided.
• bank_account_number is ignored if iban is present.
• bank_account_bic is preferred over routing_number when both are provided.
• routing_number is ignored if bank_account_bic is present.

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