Replace use of `convertStringToHex`/`convertHexToString` with `stringToHex`/`hexToString`

[mvadari]

convertStringToHex and convertHexToString are deprecated functions: https://github.com/XRPLF/xrpl.js/blob/main/packages/xrpl/src/utils/stringConversion.ts

These functions are really only still used in tests and snippets, so this is a pretty low priority issue. Just leaving it here as a reminder.