[clang] [clang][NFC] order C++ standards in reverse in release notes (PR #104866)

[via cfe-commits]

h-vetinari wrote:

Arguably the sections for C++17 and C++14 could even be deleted, since support has been declared complete as of clang 19
https://github.com/llvm/llvm-project/blob/6e3026883d77124e32a2a7be72c3361fba3e7457/clang/docs/ReleaseNotes.rst?plain=1#L194-L196

https://github.com/llvm/llvm-project/pull/104866