Do not allow older versions of Confluence to run against newer Confluence homes

Once a Confluence instance has been upgraded to a new version, it is not expected that an older version of the software will still work. However, Confluence contains no check that the home directory it is running against has not been upgraded beyond the software. This allows customers to "downgrade" to an older version and risk things going horribly wrong. Usually this happens after a partially failed upgrade. Most of the time an older version of Confluence will "pretty much" work against an upgraded instance, but there is no guarantee of that. When things do go wrong, there's nothing logged to tell the customer or support staff to tell them about the mismatched home and software versions which unnecessarily complicates matters.

Ideally Confluence would refuse to start up if the build number in the home directory is higher than the software's build number. This will infuriate customers who've upgraded unsuccessfully, but at least they will be correctly forced to revert to a backup, rather than risking unknown corruption of data.