[MIG] Migrate currency_rate_update to V8.

I'm talking about having:
```
services_mapping = {
    'ECB_getter': ECB_supported_currency_array,
    'RO_BNR_getter': RO_BNR_supported_currency_array,
...
}
```
And here use:
```
currency_list = services_mapping[self.service]
```