Reminder emails generate fatal error when triggered via drush cron

Problem/Motivation

When drush is used to trigger cron the reminder emails are causing a fatal error.

LogicException: Render context is empty, because render() was called outside of a renderRoot() or renderPlain() call. Use renderPlain()/renderRoot() or #lazy_builder/#pre_render instead. in Drupal\Core\Render\Renderer->doRender() (line 244 of /app/web/core/lib/Drupal/Core/Render/Renderer.php).

Steps to reproduce

Set a registration reminder
Run cron

Proposed resolution

Replace render with renderPlain when creating markup for emails.

Remaining tasks

Review patch

User interface changes

n/a

API changes

n/a

Data model changes

n/a