[libc++][doc] Update spaceship status page

    * `operator<=>` for `iota_view::iterator` was enabled in 8320017b
    * 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