Backport #28205 by @CodeShakingSheep
In the [docs for email
setup](https://docs.gitea.com/administration/email-setup)
`mailer.IS_TLS_ENABLED` is mentioned which was replaced by
`mailer.PROTOCOL` in release 1.18.0 according to
https://blog.gitea.com/release-of-1.18.0/ . This change wasn't reflected
in the docs for email setup. I just replaced the deprecated mailer
setting.
Co-authored-by: CodeShakingSheep <19874562+CodeShakingSheep@users.noreply.github.com>
(cherry picked from commit 5ed0eefc9a)
