libstdc++: Use 202100L as feature test check for C++23

libstdc++: Use 202100L as feature test check for C++23

I noticed that our C++23 features were not being defined when using
Clang 16 with -std=c++2b, because it only defines __cplusplus=202101L
but <bits/version.h> uses 202302L since my r14-3252-g0c316669b092fb
change.

This changes <bits/version.h> to use 202100 instead of the final 202302
value so that we support Clang 16's -std=c++2b mode.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (stds): Use >= 202100 for C++23
	condition.
	* include/bits/version.h: Regenerate.
	* include/std/thread: Use > C++20 instead of >= C++23 for
	__cplusplus condition.