Split off from #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.