MDN Requests are not honoured

[miaulalala]

Steps to reproduce

Try to send MDN mail and see that it doesn't request a read receipt

Expected behavior

MDN request is added to the mail

Actual behavior

MDN request is not added to the mail as the value is always false:

https://github.com/nextcloud/mail/blob/7109f998fa34e1920b917c1fb1e8859b957ba112/lib/Service/MailTransmission.php#L323-L336

Mail app version

3.6.0-alpha3

Mailserver or service

No response

Operating system

No response

PHP engine version

PHP 8.2

Web server

Apache (supported)

Database

MySQL

Additional info

No response