Force the XWikiUsers class document to use 2.1 syntax

A better solution for XWIKI-8033 is to force the document to use 2.1 syntax instead of the currently default syntax. As pointed out by Thomas, a corner case when the previous fix is not valid is when the currently default syntax is set by the user to 1.0.

Even though it is easily fixable by the user (by changing the configuration to use as default syntax 2.1), it might be better if we do it in the code.