WG21-P2637R3 Member visit

INCREASED feature-test macros, note that __cpp_lib_format has repeatedly increased through 202304L (#3799 WG21-P2510R3), 202305L (#3813 WG21-P2757R3), 202306L (#3808 WG21-P2637R3), 202603L (#4181 WG21-P2918R2):

#define __cpp_lib_format  202306L // <== NOTE SPECIAL INTERACTIONS
#define __cpp_lib_variant 202306L

Note: We are not yet accepting PRs for C++26 features.