Stop rejecting NS descriptors when microdescs are the default
When we merged #6769 (moved), it exposed a bug in router_get_routerlist(), which checks full router descriptors against the usable consensus flavour. This check fails for all downloaded descriptors when the usable flavour is microdescriptor.
Directory servers were somewhat immune to this issue, because they still put the descriptors that failed this check in old_routers. But this still might have caused subtle bugs down the track.