Update Mantis docs to reflec that we don't work with current versions of Mantis

Mantis 1.2 was released on 22 February. Because of JRA-19558 we no longer work with Mantis. At the very least we need to update http://confluence.atlassian.com/display/JIRA/Importing+Data+From+Mantis to make clear that we only support older versions of Mantis.

We also need to update the product tour - http://www.atlassian.com/software/jira/tour/mantis-importer.jsp - to be more specific about the Mantis version support.