Libstdc++: Update C++23 status table.
The libstdc++ documentation now reflects the C++23 features included in GCC 16.
The libstdc++ documentation has been updated to reflect the new C++23 features available in GCC 16, including one feature that was backported to GCC 15.3. This update ensures that the documentation accurately reflects the current state of C++23 support in libstdc++.
In Details
This commit updates doc/xml/manual/status_cxx2023.xml and regenerates doc/html/manual/status.html to reflect the current status of C++23 features in libstdc++. The update incorporates new features in GCC 16 and a backported feature in GCC 15.3.
For Context
This commit updates the libstdc++ (the C++ standard library for GCC) documentation to reflect the current status of C++23 features. The documentation includes a table that indicates which C++23 features are supported and to what extent. This update ensures that users can easily determine the level of C++23 support provided by libstdc++.