[MIG] Migrate currency_rate_update to V8.

@nbessi 
I still have to review the security files, waiting for your opinion what needs to change to comply with the OCA rules (being my first big contribution).
Instead of using `strptime` and `strftime`, you can use new API 8.0 date helpers:
`fields.Date.from_string()`
`fields.Datetime.to_string()`

It will use DEFAULT_SERVER_DATE_FORMAT