Google VR SDK only works with Android 7 ???!

Perhaps I’m doing something wrong, but in both 5.6 and 2017.2b1 I am unable to build an GVR project to a minimum Android API of 23 or less. It seems to want 24 (7.0 or Nougat).

Am I wrong or will this basically not work on most Android phones that otherwise work quite well with older VR SDKs? Or is it a Unity bug? Or do I need to update my Java SDK (which Unity said did for me anyway)?

Note: I don’t care about Daydream controller support. I’m trying to get a normal Cardboard panoramic VR app which has worked for over a year working on the latest stuff.

It says in the docs that the minimum API is 21, not 23: