[libc++][flat_set] LWG3751: Missing feature macro for flat_set

Implemented  in LLV21:  7013b51

Closes  #105021

Author: H-G-Hristov

libcxx/docs/Status/Cxx23Issues.csv

@@ -209,7 +209,7 @@
 "`LWG3746 <https://wg21.link/LWG3746>`__","``optional``'s spaceship with ``U`` with a type derived from optional causes infinite constraint meta-recursion","2022-11 (Kona)","|Complete|","17",""
 "`LWG3747 <https://wg21.link/LWG3747>`__","``ranges::uninitialized_copy_n``, ``ranges::uninitialized_move_n``, and ``ranges::destroy_n`` should use ``std::move``","2022-11 (Kona)","","",""
 "`LWG3750 <https://wg21.link/LWG3750>`__","Too many papers bump ``__cpp_lib_format``","2022-11 (Kona)","|Partial|","","Only ``__cpp_lib_format_ranges`` is fully implemented"
-"`LWG3751 <https://wg21.link/LWG3751>`__","Missing feature macro for ``flat_set``","2022-11 (Kona)","","",""
+"`LWG3751 <https://wg21.link/LWG3751>`__","Missing feature macro for ``flat_set``","2022-11 (Kona)","|Complete|","21",""
 "`LWG3753 <https://wg21.link/LWG3753>`__","Clarify entity vs. freestanding entity","2022-11 (Kona)","","",""
 "`LWG3754 <https://wg21.link/LWG3754>`__","Class template expected synopsis contains declarations that do not match the detailed description","2022-11 (Kona)","|Nothing To Do|","",""
 "`LWG3755 <https://wg21.link/LWG3755>`__","``tuple-for-each`` can call ``user-defined`` ``operator,``","2022-11 (Kona)","|Complete|","17",""