[CMake] Warn when the version of CMake is older than 3.31.0 (#204203)

This is a preparation to raise the minimum required CMake version to
3.31.0 in LLVM 24.

See
https://discourse.llvm.org/t/rfc-raising-minimum-required-cmake-version-to-3-31/91086.

GitOrigin-RevId: e192ec42b426f6ffbc25419238a90f3c59088587