MediaWiki 1.21/wmf12

The latest version (labeled "1.21wmf12") of MediaWiki, the software that powers Wikipedia and its sister sites, is being deployed to all Wikimedia sites in stages, starting on Monday, 18 March 2013 (see the roadmap).

We will continue to deploy the latest software every two weeks. This is intended to minimize disruption and possible issues, and make it easier to identify the cause of problems, since the possibly problematic code will be much more recent.