Update currency conversion rates

[markbrough]

Currently, currency conversion is using this set of scripts and rates, which are quite out of date:
https://github.com/codeforIATI/IATI-Stats/tree/main/helpers/currency_conversion

We should switch to using imf-exchangerates instead. There are annual rates here (also e.g. annual rates), though they are in a different shape:
https://github.com/codeforiati/imf-exchangerates