Mail Config: Updated how TLS is configured

After full review of current MAIL_ENCRYPTION usage in laravel and
smyfony mailer, this updates the options in BookStack to be simplified
and specific in usage:

- Removed mail.mailers.smtp.encryption option since it did not actually
  affect anything in the current state of dependancies.
- Updated MAIL_ENCRYPTION so values of tls OR ssl will force-enable tls
  via 'scheme' option with laravel passes to the SMTP transfport, which
  Smyfony uses as an indicator to force TLS.

When MAIL_ENCRYPTION is not used, STARTTLS will still be attempted by
symfony mailer.
Updated .env files to refer to BookStack docs (which was updated for
this) and to reflect correct default port.
Related to #4342

app/Config/mail.php

@@ -8,6 +8,10 @@
  * Do not edit this file unless you're happy to maintain any changes yourself.
  */
 
+// Configured mail encryption method.
+// STARTTLS should still be attempted, but tls/ssl forces TLS usage.
+$mailEncryption = env('MAIL_ENCRYPTION', null);
+
 return [
 
     // Mail driver to use.
@@ -27,9 +31,9 @@ return [
     'mailers' => [
         'smtp' => [
             'transport' => 'smtp',
+            'scheme' => ($mailEncryption === 'tls' || $mailEncryption === 'ssl') ? 'smtps' : null,
             'host' => env('MAIL_HOST', 'smtp.mailgun.org'),
             'port' => env('MAIL_PORT', 587),
-            'encryption' => env('MAIL_ENCRYPTION', 'tls'),
             'username' => env('MAIL_USERNAME'),
             'password' => env('MAIL_PASSWORD'),
             'verify_peer' => env('MAIL_VERIFY_SSL', true),