feat: update currency handling

Updated the way how we handle the currency in the system. Now instead of directly getting the user's currency or the default one, we use the getRuntimeCurrency() method. This method is likely to return the correct currency even if the user did not set it or if there is no default currency set in the system. This change is applied in several files including index.php, CurrencySwitch.php, and LangCurrencySwitch.php. Additionally, updated composer.json to use version 2.2 of quiqqer/currency. Removed unused code.