Mail: Removed custom symfony/mailer fork

Moved to standard symfony mailer now that my patches have been
upstreamed. This changes the config to work with the symfony option,
following the same overall logic.
Also updated testing to allow test runs via mulitple custom env options.

Closes #5636

app/Config/mail.php

@@ -11,6 +11,7 @@
 // Configured mail encryption method.
 // STARTTLS should still be attempted, but tls/ssl forces TLS usage.
 $mailEncryption = env('MAIL_ENCRYPTION', null);
+$mailPort = intval(env('MAIL_PORT', 587));
 
 return [
 
@@ -33,13 +34,13 @@ return [
             'transport' => 'smtp',
             'scheme' => null,
             'host' => env('MAIL_HOST', 'smtp.mailgun.org'),
-            'port' => env('MAIL_PORT', 587),
+            'port' => $mailPort,
             'username' => env('MAIL_USERNAME'),
             'password' => env('MAIL_PASSWORD'),
             'verify_peer' => env('MAIL_VERIFY_SSL', true),
             'timeout' => null,
             'local_domain' => null,
-            'tls_required' => ($mailEncryption === 'tls' || $mailEncryption === 'ssl'),
+            'require_tls' => ($mailEncryption === 'tls' || $mailEncryption === 'ssl' || $mailPort === 465),
         ],
 
         'sendmail' => [