libstdc++: Adjust value of __cpp_lib_constrained_equality for C++20

libstdc++: Adjust value of __cpp_lib_constrained_equality for C++20

The P3379R0 bump to __cpp_lib_constrained_equality relates to changes
that only affect std::expected, so there's no reason to define the
updated value in C++20.

This change restores the previous value (202403) for C++20, and only
uses the new value (202411) for C++23 and later.

Also remove the TODO comments, because I correctly predicted that the
final value would be 202411.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (constrained_equality): Only define
	as 202411 for C++23 and later, use 202403 for C++20.
	* include/bits/version.h: Regenerate.
	* testsuite/20_util/expected/equality_constrained.cc: Remove
	TODO comment.