generatetoaddress extracts parameters using the older request.params[] API, while its sibling generatetodescriptor already uses self.Arg<>() (since c00000df16). This migrates all three parameter extractions in generatetoaddress to the self.Arg<> API for consistency.
As a side-effect, negative maxtries values (e.g. -1) are now correctly rejected with “JSON integer out of range” instead of silently wrapping to UINT64_MAX via implicit conversion.