`rawissueasset` (and other issuance RPCs) should not allow issuing asset from a legacy output

In https://github.com/ElementsProject/elements/blob/18fadbafcceff70a7d328ca739e09004dc51e37a/src/confidential_validation.cpp#L282-L284 we check, when issuing assets, that a transaction has witness data attached to it, which can result in confusing errors when users try to issue assets starting from legacy outputs.

In general we want to discourage legacy outputs anyway except when absolutely necessary (e.g. when using `signmessage`, since we do not yet have a BIP322 implementation for Elements), since they are more costly to the user, addresses have worse checksumming properties, and they are more complex for implementors to work with. Narrowing this RPC will nudge us in that direction while also replacing a confusing footgun with a clear error message.