Use text_format for email template config to allow HTML

Problem/Motivation

Entering HTML tags in the email templates doesn't work, and the fields are textareas, meaning that the fields don't use my CKEditor editor or configuration.

Steps to reproduce

Drupal 10.4
PHP 8.3
Using Drupal Symfony Mailer.

Proposed resolution

Use text_format form elements, and accept HTML.

Remaining tasks

User interface changes

API changes

Data model changes