[libc++] Post-LLVM19-release docs cleanup (#99667)

This patch removes obsolete status pages for projects that were
completed: LLVM 18 release, C++20 Ranges and Spaceship support.

Co-authored-by: Hristo Hristov <[email protected]>

Author: H-G-Hristov

libcxx/docs/ReleaseNotes.rst

@@ -4,7 +4,6 @@
 .. toctree::
     :hidden:
 
-    ReleaseNotes/18
     ReleaseNotes/19
 
 .. The release notes are in versioned files, but we make sure to keep publishing

libcxx/docs/ReleaseNotes/18.rst

@@ -40,8 +40,7 @@ and C++26 features.
 
 Experimental support for the time zone database has progressed.
 
-Work on the ranges support has progressed. See
-:ref:`ranges-status` for the current status.
+Work on the ranges support has progressed.
 
 Work on the experimental C++17 Parallel STL has progressed. See
 :ref:`pstl-status` for the current status.