add mail transport as deprecated

Author: mykiwi

reference/configuration/swiftmailer.rst

@@ -59,7 +59,7 @@ The exact transport method to use to deliver emails. Valid values are:
 
 * smtp
 * gmail (see :doc:`/email/gmail`)
-* mail
+* mail (deprecated in SwiftMailer since version 5.4.5)
 * sendmail
 * null (same as setting `disable_delivery`_ to ``true``)