Replace/remove references to swiftmailer module

Describe your bug or feature request.

With swiftmailer being unsupported due to a security issue. (See Swift Mailer - Moderately critical - Access bypass - SA-CONTRIB-2024-006), we probably don't want to advertise it any more in Commerce.

I've found two references to the module:

- \Drupal\commerce\MailHandlerInterface in the class doc block.
- \Drupal\commerce_order\Form\OrderTypeForm::form where there is an actual link to the d.o project page.

If we do a 1-on-1 replacement instead of removing those mentions altogether, I see two options:

- https://www.drupal.org/project/symfony_mailer_lite
- https://www.drupal.org/project/symfony_mailer

I think the latter should have the preference here, but n=1.