Fix Call to deprecated method renderPlain()

[RobinHoutevelts]

Description

When scanning the project using upgrade_status the follow warnings were found:

CONTRIBUTED PROJECTS
--------------------------------------------------------------------------------
wmmailable
Scanned on Tue, 04/08/2025 - 16:51.

2 errors found. 2 warnings found.

modules/contrib/wmmailable/src/MessageBuilder.php:
+---------+------+--------------------------------------------------------------
| status  | line | message                                                      
+---------+------+--------------------------------------------------------------
| Fix now | 50   | Call to deprecated method renderPlain() of interface
Drupal\Core\Render\RendererInterface. Deprecated in
drupal:10.3.0 and is removed from drupal:12.0.0. Use
Drupal\Core\Render\RendererInterface::renderInIsolation() instead.

+---------+------+--------------------------------------------------------------
modules/contrib/wmmailable/src/Plugin/Mail/PlainMailableFormatter.php:
+---------+------+--------------------------------------------------------------
| status  | line | message                                                      
+---------+------+--------------------------------------------------------------
| Fix now | 87   | Call to deprecated method renderPlain() of interface
Drupal\Core\Render\RendererInterface. Deprecated in
drupal:10.3.0 and is removed from drupal:12.0.0. Use
Drupal\Core\Render\RendererInterface::renderInIsolation() instead.