[libc++][NFC] Add missing upcoming removal to 19 release notes

This upcoming removal was recorded in the release notes for LLVM 19
as part of the deprecation of the C++20 synchronization library in
pre-C++20 modes, but wasn't included in "Upcoming Deprecations and Removals".

Author: ldionne

libcxx/docs/ReleaseNotes/19.rst

@@ -140,6 +140,8 @@ LLVM 20
 - The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to enable
   the safe mode will be removed in LLVM 20.
 
+- The C++20 synchronization library will be removed entirely in language modes prior to C++20 in LLVM 20.
+
 LLVM 21
 ~~~~~~~
 TODO

libcxx/docs/ReleaseNotes/20.rst

@@ -53,6 +53,8 @@ Deprecations and Removals
 - TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to enable
   the safe mode will be removed in LLVM 20.
 
+- TODO: The C++20 synchronization library will be removed entirely in language modes prior to C++20 in LLVM 20.
+
 
 Upcoming Deprecations and Removals
 ----------------------------------