The field value updater does not work on XWiki Enterprise 5.3

Farm tools module has a script that allows updating values of fields, FarmTools.FarmFieldValueUpdater, which does not work on 5.3 because it uses the method

$services.wiki.getAllIds()

which is only avaiable from XWiki 6.2 onwards.

We should change this script to use a method to get all wikis which is there in the first version of $services.wiki, so that the tools are compatible with XWiki versions as old as possible (5.3 basically).