Adopt RendererInterface::renderInIsolation() instead of renderPlain()

Problem

We need to move to using renderInIsolation() instead of renderPlain().

Major - Call to deprecated method renderPlain() of class Drupal\Core\Render\Renderer: in drupal:10.3.0 and is removed from drupal:12.0.0. Use \Drupal\Core\Render\RendererInterface::renderInIsolation() instead.
in tests/src/TestHelperTrait.php (in context of class Drupal/Tests/custom_elements/Functional/CustomElementsRenderMarkupTest):39

see https://www.drupal.org/node/3407994 →

Proposed resolution

Use deprecation-helpere to maintain BC.