Enable v3 renew only when clusterVersion >= 3.6

The v3 renew isn't implemented at all on 3.5, so it can only be
enabled when the cluster version >= 3.6.

server/etcdserver/v3_server.go

@@ -276,7 +276,10 @@ func (s *EtcdServer) LeaseRevoke(ctx context.Context, r *pb.LeaseRevokeRequest)
 }
 
 func (s *EtcdServer) LeaseRenew(ctx context.Context, r *pb.LeaseKeepAliveRequest) (*pb.LeaseKeepAliveResponse, error) {
-	if s.Cfg.ExperimentalEnableLeaseV2Renew {
+	cv := s.ClusterVersion()
+	// The v3 renew isn't implemented at all on 3.5, so the v3 renew can only
+	// be used when clusterVersion >= 3.6.
+	if s.Cfg.ExperimentalEnableLeaseV2Renew || cv == nil || version.LessThan(*cv, version.V3_6) {
 		resp := &pb.LeaseKeepAliveResponse{ID: r.ID, Header: s.newHeader()}
 
 		var err error