drupal_render is deprecated so it has to be removed

Problem/Motivation

According to https://api.drupal.org/api/drupal/core!includes!common.inc/function/drup... drupal_render is deprecated. xmlsitemap still uses drupal_render in its .module file.

Proposed resolution

change it to \Drupal::service('renderer')->render($elements, $is_recursive_call);

Remaining tasks

Create a patch.
Review.

User interface changes

API changes