std::istrstream is deprecated

[KrisThielemans]

It is used in KeyParser experimental/utilities/mode.cxx

https://en.cppreference.com/w/cpp/io/istrstream.html

istrstream has been deprecated since C++98 and removed since C++26. std::istringstream, std::ispanstream(since C++23), and boost::iostreams::array_source are the recommended replacements.