Update dbus-advertised screencast source types

The upcoming version of OBS Studio (along with any other application that cares) uses the org.freedesktop.portal.ScreenCast dbus interface to check which screencast modes the portal backend supports. Previous versions didn't check, which is, as far as I can tell, the reason this line wasn't changed until now; the functionality is implemented and has been working just fine.

That being said, I recognize that trying to use both whole monitor and per-window screencasting at once can lead to other issues, but either mode on its own seems to work as intended.