Set the XWiki 2.1 syntax in the Sandbox

For a long time now (XWiki 3.4), XWiki Syntax 2.1 is the default syntax in XWiki. But in Sandbox, it is still the 2.0 which is used.

This is very bad since it should be the first page that the user edits, where she can play with the syntax and so having the bad feeling that XWiki Syntax 2.1 does not work.