Use "currency conversion fee" not "foreign exchange fee"

[aheckler]

This issue is a result of this P2 post: pbUcTB-5Uz-p2

TLDR: We should strongly consider moving away from using "foreign exchange fee" when referring to the fee itself, and instead use "currency conversion fee." We can still use "foreign exchange" when talking about the current rate at which one currency converts to another. This matches Stripe's usage.

Instances of "foreign exchange" in code:

https://github.com/search?q=repo%3AAutomattic%2Fwoocommerce-payments+%22foreign+exchange%22&type=code