Elem::centroid() -> Elem::vertex_average()
Elem::centroid() -> Elem::vertex_average()

libMesh deprecated Elem::centroid() due to its misleading name - that
method was actually returning a vertex average for most elements, even
when that average was not the true centroid.

For this application it looks like vertex_average() (which is faster
than a true_centroid() calculation in general) should work correctly;
let's request it explicitly.

Refs #13921

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