MailManagerDecorator renders related plugins unavailable

Problem/Motivation

The MailManagerDecorator uses an out-of-date definition of the MailManager plugin manager definition.
Since Drupal 10.3.0 the Mail plugins using attributes instead of annotations, so when this module gets enabled, the related plugins with attribute-only definition become unavailable.

Related core change: https://git.drupalcode.org/project/drupal/-/commit/a2c8988da4ca9bc96729c...

Steps to reproduce

• Install Drupal core.
• Check available plugins by running drush ev "print_r(array_keys(\Drupal::service('plugin.manager.mail')->getDefinitions()));".
• Enable view_usernames module.
• Run drush cr.
• Run drush ev "print_r(array_keys(\Drupal::service('plugin.manager.mail')->getDefinitions()));" command again to check for the available plugins.

While the module is not enabled the command will show the core defined mail plugin IDs, but after enabling the module the list will be empty.

Proposed resolution

Update MailManagerDecorator to be in sync with the MailManager plugin manager's definition.

Remaining tasks

Create merge request.