etcdserver: update etcdserver to use the new raft module go.etcd.io/raft/v3

Just replaced all go.etcd.io/etcd/raft/v3 with go.etcd.io/raft/v3
under directory server.

Signed-off-by: Benjamin Wang <wachao@vmware.com>

server/etcdserver/api/rafthttp/util_test.go

@@ -23,7 +23,7 @@ import (
 	"testing"
 
 	"go.etcd.io/etcd/api/v3/version"
-	"go.etcd.io/etcd/raft/v3/raftpb"
+	"go.etcd.io/raft/v3/raftpb"
 
 	"github.com/coreos/go-semver/semver"
 )