vendor: upgrade gRPC Go to v1.24.0

Picking up some performance improvements and bug fixes.

https://github.com/grpc/grpc-go/releases/tag/v1.24.0

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

vendor/google.golang.org/grpc/service_config.go

@@ -310,6 +310,14 @@ func parseServiceConfig(js string) (*ServiceConfig, error) {
 			}
 			break
 		}
+		if sc.lbConfig == nil {
+			// We had a loadBalancingConfig field but did not encounter a
+			// supported policy.  The config is considered invalid in this
+			// case.
+			err := fmt.Errorf("invalid loadBalancingConfig: no supported policies found")
+			grpclog.Warningf(err.Error())
+			return nil, err
+		}
 	}
 
 	if rsc.MethodConfig == nil {