C2: Migration of mailing configuration

[NicoDucou]

In Chamilo 1.11.x the mailing configuration is in the file app/config/mail.conf.php
This setting is not automatically transferred to .env file in the following directive :

###> symfony/mailer ###
#MAILER_DSN=*******
###< symfony/mailer ###

There are to many different cases to try to manage it automatically so it would probably be better to indicate it in the migration process as a manual action.