Forbid FV boundary conditions on aux variables since we no longer use that, refs #15836
Forbid FV boundary conditions on aux variables since we no longer use that, refs #15836

sapitts/moose,milljm/moose,sapitts/moose,jessecarterMOOSE/moose,andrsd/moose,laagesen/moose,laagesen/moose,harterj/moose,lindsayad/moose,sapitts/moose,lindsayad/moose,harterj/moose,andrsd/moose,idaholab/moose,dschwen/moose,harterj/moose,andrsd/moose,idaholab/moose,bwspenc/moose,dschwen/moose,bwspenc/moose,lindsayad/moose,lindsayad/moose,andrsd/moose,idaholab/moose,bwspenc/moose,harterj/moose,jessecarterMOOSE/moose,milljm/moose,laagesen/moose,dschwen/moose,idaholab/moose,laagesen/moose,laagesen/moose,milljm/moose,jessecarterMOOSE/moose,jessecarterMOOSE/moose,idaholab/moose,harterj/moose,jessecarterMOOSE/moose,milljm/moose,bwspenc/moose,sapitts/moose,andrsd/moose,lindsayad/moose,dschwen/moose,dschwen/moose,milljm/moose,sapitts/moose,bwspenc/moose