Merge pull request #15563 from masterleinad/require_cxx17

Require C++17

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

diff --cc include/deal.II/grid/grid_tools.h
Simple merge

diff --cc include/deal.II/non_matching/fe_values.h
Simple merge

diff --cc include/deal.II/non_matching/quadrature_generator.h
Simple merge

diff --cc source/non_matching/fe_values.cc
Simple merge

diff --cc source/non_matching/quadrature_generator.cc
Simple merge