Use NewDynamicRESTMapper instead of NewDisoveryRESTMapper

The latter isn't remommended and was removed by
kubernetes-sigs/controller-runtime#2611
in controller-runtime v0.17.0.

Signed-off-by: Tom Pantelis <tompantelis@gmail.com>

main.go

@@ -173,7 +173,7 @@ func main() {
 		Cache: cache.Options{
 			DefaultNamespaces: map[string]cache.Config{namespace: {}},
 		},
-		MapperProvider: apiutil.NewDiscoveryRESTMapper,
+		MapperProvider: apiutil.NewDynamicRESTMapper,
 	})
 	if err != nil {
 		log.Error(err, "unable to start manager")