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

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

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

tests/mssql.ini.tmpl

@@ -58,7 +58,7 @@ PATH = tests/{{TEST_TYPE}}/gitea-{{TEST_TYPE}}-mssql/data/attachments
 
 [mailer]
 ENABLED = true
-MAILER_TYPE = dummy
+PROTOCOL = dummy
 FROM = mssql-{{TEST_TYPE}}-test@gitea.io
 
 [service]

tests/mysql.ini.tmpl

@@ -58,7 +58,7 @@ SSH_TRUSTED_USER_CA_KEYS = ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABgQCb4DC1dMFnJ6pXW
 
 [mailer]
 ENABLED = true
-MAILER_TYPE = dummy
+PROTOCOL = dummy
 FROM = mysql-{{TEST_TYPE}}-test@gitea.io
 
 [service]

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]

tests/sqlite.ini.tmpl

@@ -55,7 +55,7 @@ PATH = tests/{{TEST_TYPE}}/gitea-{{TEST_TYPE}}-sqlite/data/attachments
 
 [mailer]
 ENABLED     = true
-MAILER_TYPE = dummy
+PROTOCOL = dummy
 FROM        = sqlite-{{TEST_TYPE}}-test@gitea.io
 
 [service]