[libc++] Reword release note section about future releases (#138544)

For several releases, we had a section in the release notes that was
called "Upcoming Deprecations and Removals". That section was used to
advertize breaking changes in future releases as opposed to ones in the
current release.

However, the way this section was worded and organized made it unclear
what release these announcements related to. This patch rewords that
section of the release notes to make it less ambiguous and moves items
that aren't done yet (but relate to the ongoing release) to a different
section with a TODO.

libcxx/docs/ReleaseNotes/21.rst

@@ -80,16 +80,18 @@ Deprecations and Removals
 - The ``_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT`` has been removed, making ``std::__libcpp_verbose_abort``
   unconditionally ``noexcept``.
 
+- TODO: The non-conforming extension ``packaged_task::result_type`` has been removed in LLVM 21.
+
 Potentially breaking changes
 ----------------------------
 
 - The implementation of ``num_put::do_put`` has been replaced to improve the performance, which can lead to different
   output when printing pointers.
 
-Upcoming Deprecations and Removals
-----------------------------------
+Announcements About Future Releases
+-----------------------------------
 
-LLVM 21
+LLVM 22
 ~~~~~~~
 
 - The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22,
@@ -101,13 +103,6 @@ LLVM 21
   If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most
   out of libc++.
 
-- Non-conforming extension ``packaged_task::result_type`` will be removed in LLVM 21.
-
-LLVM 22
-~~~~~~~
-
-- TODO
-
 
 ABI Affecting Changes
 ---------------------