New code for unrecognized currencies

As of API version 2021-07, the GraphQL Admin API returns XXX in response to an unrecognized currency code. XXX is the ISO code used to denote transactions that involve no currency.

In mutations, a CurrencyCode input value of XXX is invalid. XXX can only be returned as a result and not as an input value.

This change prevents order pages from breaking if payments include a currency code that's not in the list of supported values for the CurrencyCode object.