Improve error message for unsupported SCRAM mechanism when authenticating with local.__system user

SERVER-46399 removed SCRAM-SHA-1 as an implicit auth mechanism for intra-cluster authentication - when an attempt is now made to authenticate using SCRAM-SHA-1 with the local.__system user, the following misleading error is reported:

 “AuthenticationFailed: It is not possible to authenticate as the __system user on servers started without a --keyFile parameter”

We should improve the error message to be less confusing by failing the request earlier, e.g. in this block:

https://github.com/mongodb/mongo/blob/d5f5bf69042dbef818e2d0adf84799a6a6d33aa9/src/mongo/db/auth/sasl_scram_server_conversation.cpp#L183