Update features_cpp23.yaml for P1264 in GCC

I confirm that libstdc++ already implements the expected behaviour.

Signed-off-by: Jonathan Wakely <cxx@kayari.org>

Author: jwakely

features_cpp23.yaml

@@ -1010,7 +1010,7 @@ features:
     paper: P1264
     lib: true
     support:
-      - GCC ?
+      - GCC
       - Clang 9
       - MSVC
       - Xcode 12