[libc++][doc] Update spaceship status page
* `operator<=>` for `iota_view::iterator` was enabled in
8320017b79eb8
* Removed P2405R0 which was not accepted and seems inactive
(https://github.com/cplusplus/papers/issues/1075)
* Added the previously missing `operator==` for `filesystem::space_info`
to the tracking list.
* Updated the "Assignee" for `string_view`, `string` as Mark de Wever
mentioned he is working on them in Discord
* Updated the status of the items for which I sent review requests
yesterday.
Reviewed By: #libc, Mordante
Differential Revision: https://reviews.llvm.org/D130855