Demote About app's VirtualUriMapping subapp

Since 5.5.6, VirtualUriMappings have a new API and are stored in a registry (and bring up YAML mappings)
mappings implementing the new API, as well as YAML mappings do not show in the old subapp of the About app. (where it was also mis-placed, see related UX ticket).

The source of truth for URI mappings is now the Definitions app.

As in 5.6 we updated all our default mappings in magnolia modules; that subapp may render as empty. So we consider two things:

• deprecate the subapp with immediate effect
• hide that subapp, by removing its startup from the AboutApp class (can still be started manually by location)