And constrain the new `operator==` since C++26.
This patch implements parts of P2165R4, P2944R3, and a possibly improved
resolution of LWG3882. Currently, libstdc++ and MSVC STL constrain the
new overloads in the same way.
Also set feature-test macro `__cpp_lib_constrained_equality` and add
related release note, as P2944R3 will completed with this patch.
Fixes#136765Fixes#136770Fixes#105424