Update email-setup.en-us.md (#26068) (#26166)

Backport #26068 by @felixvictor

The setting `MAILER_TYPE` is deprecated.
According to the config cheat sheet, it should be `PROTOCOL`.

Co-authored-by: Felix Victor <felix.victor.na@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

tests/pgsql.ini.tmpl

@@ -59,7 +59,7 @@ PATH = tests/{{TEST_TYPE}}/gitea-{{TEST_TYPE}}-pgsql/data/attachments
 
 [mailer]
 ENABLED = true
-MAILER_TYPE = dummy
+PROTOCOL = dummy
 FROM = pgsql-{{TEST_TYPE}}-test@gitea.io
 
 [service]