Revert "node/convert: relax the check for string_view (#1222)" (#1225)

This reverts commit 62622011.

in 62622011, we wanted address the needs to use the `string_view`
converter in C++98, but that requirement was based on wrong
preconditions. `std::string_view` was introduced in C++17, and
popular standard libraries like libstdc++ and libc++ both provide
`std::string_view` when the source is built with C++17.

furthermore 62622011 is buggy. because it uses `<version>` to tell
the feature set provided by the standard library. but `<version>`
is a part of C++20. so this defeats the purpose of the change of
62622011.

Fixes #1223

include/yaml-cpp/node/convert.h

@@ -17,9 +17,8 @@
 #include <type_traits>
 #include <valarray>
 #include <vector>
-#include <version>
 
-#ifdef __cpp_lib_string_view
+#if __cplusplus >= 201703L
 #include <string_view>
 #endif
 
@@ -94,7 +93,7 @@ struct convert<char[N]> {
   static Node encode(const char* rhs) { return Node(rhs); }
 };
 
-#ifdef __cpp_lib_string_view
+#if __cplusplus >= 201703L
 template <>
 struct convert<std::string_view> {
   static Node encode(std::string_view rhs) { return Node(std::string(rhs)); }