We extracted the widgetset out of UI to its own project; now the widgetset package has changed as follows:
5.2 => info.magnolia.ui.vaadin.gwt.MagnoliaWidgetSet
5.3 => info.magnolia.widgetset.MagnoliaWidgetSet

This property is read from the magnolia.ui.vaadin.widgetset property in the magnolia.properties file.

This means that for upgrading from 5.2 to 5.3, we need to make sure people update this property value in their setup, so we will add a condition in version handler for that.