Provide an appclient.xml that does not use the legacy security subsystem

For our EE9 variant (and probably for EE 8 at some point) we need an appclient.xml that does not use legacy security.

https://github.com/wildfly/wildfly/blob/master/ee-feature-pack/common/src/main/resources/content/appclient/configuration/appclient.xml#L143