fix(admin): use gRPC address for current server in RaftListClusterServers
The old Raft implementation was returning the HTTP address
(ms.option.Master) for the current server, while peers used gRPC
addresses (peer.ConnectionString). The Admin UI's GetClusterMasters()
converts all addresses from gRPC to HTTP via GrpcAddressToServerAddress
(port - 10000), which produced a negative port (-667) for the current
server since its address was already in HTTP format (port 9333).
Use ToGrpcAddress() for consistency with both HashicorpRaft (which
stores gRPC addresses) and old Raft peers.
Fixes#8921
896114d330