[libc++] Add a release note about C++03 being frozen after LLVM 21 (#95894)

Co-authored-by: Louis Dionne <[email protected]>

Author: philnik777

libcxx/docs/ReleaseNotes/19.rst

@@ -169,8 +169,13 @@ LLVM 20
 
 LLVM 21
 ~~~~~~~
-TODO
 
+- 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 C++03 is a legacy platform, where most projects are no longer actively maintained.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+  If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most out of libc++.
 
 ABI Affecting Changes
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