Check for __cpp_lib_remove_cvref as well (#5761)

C++20 can be enabled while the C++ runtime is still much older so use the feature macro to check for it. 

For example, we are using the latest clang with c++23 on SLES, while the gcc version is 7.

Author: pfultz2

include/pybind11/detail/common.h

@@ -697,7 +697,7 @@ template <typename T>
 using remove_reference_t = typename std::remove_reference<T>::type;
 #endif
 
-#if defined(PYBIND11_CPP20)
+#if defined(PYBIND11_CPP20) && defined(__cpp_lib_remove_cvref)
 using std::remove_cvref;
 using std::remove_cvref_t;
 #else