Deprecate `s2s_use_starttls: required_trusted`

The functionality can be now obtained by using
`s2s_use_starttls: required` with `mod_s2s_dialback`
being unloaded.

ejabberd.yml.example

@@ -231,7 +231,7 @@ listen:
 
 ##
 ## s2s_use_starttls: Enable STARTTLS for S2S connections.
-## Allowed values are: false optional required required_trusted
+## Allowed values are: false, optional or required
 ## You must specify a certificate file.
 ##
 ## s2s_use_starttls: required

src/ejabberd_s2s.erl

@@ -726,7 +726,13 @@ opt_type(s2s_use_starttls) ->
 	(false) -> false;
 	(optional) -> optional;
 	(required) -> required;
-	(required_trusted) -> required_trusted
+	(required_trusted) ->
+	    ?WARNING_MSG("The value 'required_trusted' of option "
+			 "'s2s_use_starttls' is deprected and will be "
+			 "unsupported in future releases. Instead, "
+			 "set it to 'required' and make sure "
+			 "mod_s2s_dialback is *NOT* loaded", []),
+	    required_trusted
     end;
 opt_type(s2s_zlib) ->
     fun(B) when is_boolean(B) -> B end;