remove deprecated std::result_of

[BenWibking]

src/tasks/tasks.hpp uses std::result_of, which is deprecated in C++17 and removed in C++20. This causes lots of warnings on my machine:

In file included from /Users/benwibking/athenapk/external/parthenon/src/tasks/tasks.hpp:30:
/Users/benwibking/athenapk/external/parthenon/src/tasks/thread_pool.hpp:121:36: warning: 'result_of<(lambda at /Users/benwibking/athenapk/external/parthenon/src/tasks/tasks.hpp:420:20) ()>' is deprecated [-Wdeprecated-declarations]
    using return_t = typename std::result_of<F(Args...)>::type;
                                   ^
/Users/benwibking/athenapk/external/parthenon/src/tasks/tasks.hpp:420:12: note: in instantiation of function template specialization 'parthenon::ThreadPool::enqueue<(lambda at /Users/benwibking/athenapk/external/parthenon/src/tasks/tasks.hpp:420:20)>' requested here
      pool.enqueue([t, &ProcessTask]() { return ProcessTask(t); });
           ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.2.sdk/usr/include/c++/v1/__type_traits/result_of.h:24:34: note: 'result_of<(lambda at /Users/benwibking/athenapk/external/parthenon/src/tasks/tasks.hpp:420:20) ()>' has been explicitly marked deprecated here
template <class _Callable> class _LIBCPP_DEPRECATED_IN_CXX17 result_of;
                                 ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.2.sdk/usr/include/c++/v1/__config:862:41: note: expanded from macro '_LIBCPP_DEPRECATED_IN_CXX17'
#    define _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_DEPRECATED
                                        ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.2.sdk/usr/include/c++/v1/__config:835:49: note: expanded from macro '_LIBCPP_DEPRECATED'
#      define _LIBCPP_DEPRECATED __attribute__((deprecated))
                                                ^
4 warnings generated.

[Yurlungur]

Ping @jdolence

[pbrady]

Unless I'm misunderstanding something, this is a straightforward change to std::invoke_result_t. Is this more about dropping c++14?