[libc++] Implement P1423R3 (char8_t backward compatibility remediation)

Reviewed By: Mordante, #libc

Spies: h-vetinari, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D129195

NOKEYCHECK=True
GitOrigin-RevId: 309aed306817e5b1a302d8f757231c734cf71370
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index da76d39..138759a 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -49,6 +49,7 @@
 - P0618R0 (Deprecating <codecvt>)
 - P2418R2 (Add support for ``std::generator``-like types to ``std::format``)
 - LWG3659 (Consider ``ATOMIC_FLAG_INIT`` undeprecation)
+- P1423R3 (char8_t backward compatibility remediation)
 
 - Marked the following papers as "Complete" (note that some of those might have
   been implemented in a previous release but not marked as such):