P3379R0 Constrain std::expected equality operators

[jensmaurer]

Fixes #7416
Also fixes cplusplus/papers#2036

[jwakely]

Oh dear, it looks like LWG reviewed and approved R0 of this paper. At least this time the poll matched what was reviewed.

The only difference is that R1 says that the __cpp_lib_constrained_equality feature test macro should also be present in <expected>, so when updating [version.syn] (which is missing from this PR) it should also add ", <expected>" to the comment.

[jwakely]

There's a missing edit to [version.syn]:

Update the value of the __cpp_lib_constrained_equality feature test macro in [version.syn].

[jensmaurer]

@jwakely , fixed the missing version bump

I think we've decided before that feature-test macros are sort-of editorial, so I've added the <expected> amendment with a separate commit.