Fix #490: swap dead exchange rate API in !convert

[prakhar2000]

Fixes #490

!convert is broken because exchangeratesapi.io now requires a key (and even then the free tier only allows EUR as the base). The issue calls it !currency but the command is !convert.
Changes in Parsers/Currency Converter.js:

1. Switched to Frankfurter v1 (https://api.frankfurter.dev/v1/latest)which is free, no API key, same { base, rates } response the parser already reads
2. Uppercased currency codes so !convert 10 usd to eur actually hits rates.EUR
3. Fixed the error message since it was using regular quotes so Slack posted ${fromCurrency} literally

I hit the Frankfurter endpoint and the response shape matches. Haven't run it on a live ServiceNow instance.
Not in this PR: decimal amounts ((\d+) is also the activation regex) and checking HTTP status before JSON.parse.