[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".

NOKEYCHECK=True
GitOrigin-RevId: 45bc85603a3b96bd8bf39073c2b865aac094ba4b
2 files changed