I have tried to copy-paste securityadvisory.aspx file to \Webiste\Widgets folder and it didn't work. I have tried to copy it to \WidgetCustom folder and I have managed to see it in Configurations and add widget to Dashboard, however it just rendered content of the scurityadvisory.aspx page which is kind of "This is a marker file generated by the precompilation tool, and should not be deleted!" Then i have found compiled version in \Bin folder and copy it as well. It didn't work with this compiled version. I have gave up of this widget. I couldn't find anything in Release Notes why this widget has been removed.
the predefined widgets are allocated in the C:\Program Files (x86)\Lansweeper\Website\Widgets folder. Maybe you can copy it from the previous version (installing it in a different system) and try whether it can be executed in the newer version copying it in the Widgets or WidgetCustom folder (maybe it needs only some slight changes).