TASKMAN-24: Custom created severities/statuses aren't properly displayed when adding a new task

Starting XWiki 6.2, you can use `someObject.getValue('someProperty')` instead of `someObject.getProperty('someProperty').value`

See https://github.com/xwiki/xwiki-platform/blob/2775d1e50cbbee5feb53c68463db194425332200/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/api/Collection.java#L99