raft: generator: correct constraints on members
raft: generator: correct constraints on members

A member variable is a reference, not a pure value, so std::same_as<>
needs to be given a reference (and clanf 13 insists). However, clang
12 doesn't accept the correct constraint, so use std::convertible_to<>
as a compromise.

Closes #9642

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla