Update Gmail example (#26302)

The `IS_TLS_ENABLED` option in the `mailer` section is deprecated. This
is specified by setting `PROTOCOL=smtps`

Author: elzapp

docs/content/administration/email-setup.en-us.md

@@ -79,8 +79,7 @@ SMTP_PORT      = 465
 FROM           = [email protected]
 USER           = example.user
 PASSWD         = `***`
-PROTOCOL       = smtp
-IS_TLS_ENABLED = true
+PROTOCOL       = smtps
 ```
 
 Note that you'll need to create and use an [App password](https://support.google.com/accounts/answer/185833?hl=en) by enabling 2FA on your Google