Add language about discounted rates for special currencies in checkout

Split off from https://github.com/Automattic/wp-calypso/issues/80455

For some currencies, like IDR and MXN, we apply a 20% discount for the first year of a new purchase, then charge full price for subsequent renewals (see p7DVsv-9lB-p2).

In checkout we display this as an "Upgrade credit" but there is no upgrade happening; it is a first-term discount.

We need to update the line item language to clearly explain that.


Start in the checkout UI at the line item that displays “Upgrade credit,” then trace where that label and its supporting text are assembled. Done means the line item clearly describes the first-term discount for special currencies and indicates that subsequent renewals use the full price.

Written by the indexing model from the issue text.