Fix USDC and USDT symbols on Sepolia

[MantisClone]

Problem

Users expect the default currency symbols on Sepolia to be USDC and USDT but they're currently fUSDC and fUSDT (short for "Fake USDC" and "Fake USDT").

Considerations

Caution. The full ERC20 address is stored in request.currencyInfo. The symbol is stored in request.currency. How is request.currency used?

Definition of Done

• Test Suite showing that changing the default symbol will not break requests that are already stored