Original PR description

The message `"The rounding method must be set to 0.5 and HALF-UP"` was wrong in `pos_blackbox_be/models/pos_config.py`

```py
def _check_cash_rounding(self):
        if not self.cash_rounding:
            raise ValidationError(_("Cash rounding must be enabled"))
        if (
            self.rounding_method.rounding != 0.05
            or self.rounding_method.rounding_method != "HALF-UP"
        ):
            raise ValidationError(
                _("The rounding method must be set to 0.05 and HALF-UP")
            )
```
It should be `"The rounding method must be set to 0.05 and HALF-UP"` as indicated in that documentation :
https://www.odoo.com/documentation/17.0/applications/finance/fiscal_localizations/belgium.html?highlight=blackbox#certified-pos-system
It's the same for 18.0

I also added the field for translation into `pos_blackbox_be.pot`

opw-4862967

Forward-Port-Of: odoo/enterprise#89789
Forward-Port-Of: odoo/enterprise#87496