libstdc++: Fix missing and incorrect feature test macros [PR105269]

libstdc++: Fix missing and incorrect feature test macros [PR105269]

libstdc++-v3/ChangeLog:

	PR libstdc++/105269
	* include/bits/stl_vector.h (__cpp_lib_constexpr_vector):
	Define.
	* include/c_compatibility/stdatomic.h (__cpp_lib_stdatomic_h):
	Define.
	* include/std/optional (__cpp_lib_optional): Define new value
	for C++23.
	(__cpp_lib_monadic_optional): Remove.
	* include/std/version (__cpp_lib_constexpr_vector): Define.
	(__cpp_lib_stdatomic_h): Define.
	(__cpp_lib_optional): Define new value for C++23.
	(__cpp_lib_monadic_optional): Remove.
	* testsuite/20_util/optional/monadic/and_then.cc: Adjust.
	* testsuite/20_util/optional/requirements.cc: Adjust for C++23.
	* testsuite/20_util/optional/version.cc: Likewise.
	* testsuite/23_containers/vector/cons/constexpr.cc: Check
	feature test macro.
	* testsuite/29_atomics/headers/stdatomic.h/c_compat.cc:
	Likewise.
	* testsuite/20_util/optional/monadic/version.cc: Removed.
	* testsuite/23_containers/vector/requirements/version.cc: New test.
	* testsuite/29_atomics/headers/stdatomic.h/version.cc: New test.