build-aux: Use GNOME 49 runtime for Gtk 3 example, too

[smcv]

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@TheEvilSkeleton, was there a reason you didn't do this already in #206?

[TheEvilSkeleton]

Yes, because the window explicitly states "NEW VERSION":

libportal/portal-test/gtk3/portal-test-win.ui

Line 98 in 8cfddc0

<property name="title">Portal Test NEW VERSION!!</property>

Since it wants us to use the GTK4 frontend, I didn't bother bumping the runtime for the GTK3 version.

[smcv]

That's been there since several years before the Gtk4 example was added. I don't think it means "you should use the new version instead", I think it means something more like "this is the new version of the Gtk3 portal test (integrated into libportal), not the old version (a standalone app)".

libportal should continue to have a Gtk 3 UI for as long as it has portal-gtk3.h, so that there's an easy way to do manual testing.

[TheEvilSkeleton]

Fair. I haven't tested this MR, but if it works, perhaps we can merge it?