[libc++][doc] Updates the release notes.

Copies the not-yet-implemented items planned for removal from the
LLVM-20 to the LLVM-21 release notes. This allows to better keep track
of the status of the next release.

Author: mordante

libcxx/docs/ReleaseNotes/21.rst

@@ -79,6 +79,23 @@ Potentially breaking changes
 Upcoming Deprecations and Removals
 ----------------------------------
 
+LLVM 21
+~~~~~~~
+
+- The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22,
+  non-critical bug fixes may not be back-ported to C++03, including LWG issues. C++03 is a legacy platform, where most
+  projects are no longer actively maintained. To reduce the amount of fixes required to keep such legacy projects
+  compiling with up-to-date toolchains, libc++ will aim to freeze the status of the headers in C++03 mode to avoid
+  unintended breaking changes. See https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc for more details.
+
+  If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most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++.
+
+- The ``_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT`` macro will be removed in LLVM 21, making ``std::__libcpp_verbose_abort``
+  unconditionally ``noexcept``.
+
+- Non-conforming extension ``packaged_task::result_type`` will be removed in LLVM 21.
+
 LLVM 22
 ~~~~~~~