Migrate from Swiftmailer to Symfony Mailer

[come-nc]

We currently use https://github.com/swiftmailer/swiftmailer which is stopping development at the end of November and advising to move to Symfony Mailer.

There are rectors for this but I did not manage to get them to work on our code as-is:
https://github.com/rectorphp/rector-symfony/blob/main/config/sets/swiftmailer/swiftmailer-to-symfony-mailer.php
https://github.com/rectorphp/rector-symfony/blob/main/src/Rector/MethodCall/SwiftSetBodyToHtmlPlainMethodCallRector.php

Switching from \Swift_Message to \Symfony\Component\Mime\Email is pretty straightforward and should not affect our public interfaces in \OCP\Mail.
The transport part may be less straightforward, but both sendmail and smtp are supported by symfony mailer.