Support for CMake 3.28?

[pdimov]

Currently, CMakeLists.txt requires CMake 3.30. It would be much more convenient if it required 3.28, because 3.28 is the version on Ubuntu 24.04 LTS, and installing a newer CMake there, while possible, is still somewhat a hassle.

I tried changing 3.30 to 3.28 manually, and everything appeared to still work. I'm not sure why 3.30 is required.

[ClausKlein]

It is needed for c++20 modules!

[pdimov]

It's only needed when modules are used. When BEMAN_USE_MODULES is explicitly set to OFF, there's no need to abort. (Also see #247.)