It was replaced with the renderer service:

$rendered = \Drupal::service('renderer')->render($renderArray);

The change record is slightly out of date; the method is render rather than renderElement these days.

It was replaced with the renderer service.

$render_service = \Drupal::service('renderer');

As mpdonadio and 4k4 have pointed out in the comments, to prevent metadata from bubbling up to the main request, you should execute this in a new render context. You can do so by using the renderPlain method, rather than the usual render method (unless you're returning this from a controller, in which case use renderRoot instead).

$rendered = $render_service->renderPlain($build);