Merge pull request #17711 from bangerth/subscriptor-rename

Also rename the Subscriptor base class.

diff --cc include/deal.II/base/function.h
Simple merge

diff --cc include/deal.II/base/observer_pointer.h
Simple merge

diff --cc include/deal.II/hp/collection.h
Simple merge

diff --cc include/deal.II/matrix_free/matrix_free.templates.h
Simple merge

diff --cc include/deal.II/matrix_free/operators.h
Simple merge

diff --cc include/deal.II/multigrid/mg_transfer_global_coarsening.h
Simple merge

diff --cc source/grid/tria.cc
Simple merge