[libc++][CI] Adds a new CMake version in Docker. (#71582)

This allows testing the upcoming CMake 3.28 release in the CI. CMake
3.28 will have non-experimental support for C++20 modules. So this would
be a better CMake version for the modular builds.

The goal is to remove CMake 3.27 from the CI when the builder work
properly with 3.28.