The XWikiUser's email custom displayer does not consider the situation when the class document is in syntax 1.0

The custom displayer of the XWikiUsers class is written specifically in 2.0 syntax, when the class is initialized in oldcore.

To avoid issues, we should force the XWikiUsers document to use the currently default wiki syntax.

Any users that are migrating from a 1.0 syntax document that have added custom displayers in 1.0 (or generally older than the current default) syntax will have to update them.