C++20 support

[adayton1]

Axom and some of its dependencies make use of std::result_of, which is removed in C++20. std::invoke_result_t (added in C++17) should be used instead since Axom is now on C++17. Below are the files with std::result_of is used:

• src/axom/spin/ImplicitGrid.hpp (Use std::invoke_result_t instead of deprecated/removed std::result_of #1721)
• src/thirdparty/axom/sol.hpp
• src/thirdparty/axom/CLI11.hpp (this looks like it is already properly guarded by checking the c++ standard this file is being compiled with)

[rhornung67]

Thanks @adayton1 for pointing that out.