fix(auth): restore use of grpc.Dial

There have been a couple of reports lately around some subtle
changes in behaviour of using the new recommend method. There is
also a known bug, see grpc/grpc-go#7556.
Until the bug is fixed and we have a way forward we will revert to
calling the deprecated Dial function.

Fixes: googleapis#7556

auth/grpctransport/grpctransport.go

@@ -322,7 +322,7 @@ func dial(ctx context.Context, secure bool, opts *Options) (*grpc.ClientConn, er
 	grpcOpts = addOpenTelemetryStatsHandler(grpcOpts, opts)
 	grpcOpts = append(grpcOpts, opts.GRPCDialOpts...)
 
-	return grpc.NewClient(endpoint, grpcOpts...)
+	return grpc.Dial(endpoint, grpcOpts...)
 }
 
 // grpcKeyProvider satisfies https://pkg.go.dev/google.golang.org/grpc/credentials#PerRPCCredentials.

auth/grpctransport/pool_test.go

@@ -68,7 +68,7 @@ func TestClose(t *testing.T) {
 
 	pool := &roundRobinConnPool{}
 	for i := 0; i < 4; i++ {
-		conn, err := grpc.NewClient(l.Addr().String(), grpc.WithTransportCredentials(insecure.NewCredentials()))
+		conn, err := grpc.Dial(l.Addr().String(), grpc.WithTransportCredentials(insecure.NewCredentials()))
 		if err != nil {
 			t.Fatal(err)
 		}