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

.env.example.complete

@@ -69,23 +69,19 @@ DB_PASSWORD=database_user_password
 # certificate itself (Common Name or Subject Alternative Name).
 MYSQL_ATTR_SSL_CA="/path/to/ca.pem"
 
-# Mail system to use
-# Can be 'smtp' or 'sendmail'
+# Mail configuration
+# Refer to https://www.bookstackapp.com/docs/admin/email-webhooks/#email-configuration
 MAIL_DRIVER=smtp
-
-# Mail sending options
 MAIL_FROM=mail@bookstackapp.com
 MAIL_FROM_NAME=BookStack
 
-# SMTP mail options
 MAIL_HOST=localhost
-MAIL_PORT=1025
+MAIL_PORT=587
 MAIL_USERNAME=null
 MAIL_PASSWORD=null
 MAIL_ENCRYPTION=null
 MAIL_VERIFY_SSL=true
 
-# Command to use when email is sent via sendmail
 MAIL_SENDMAIL_COMMAND="/usr/sbin/sendmail -bs"
 
 # Cache & Session driver to use