CMake module: use 3.28 also on Ubuntu 18

Falk Baldufrequested to merge
uncommitted-changes/armar7a-2-2025-02-14 into main

This PR simplifies the CMake module. Instead of having two CMake versions (18.04: 3.21; 22.04: 3.28), we can just use 3.28.

Edited by Fabian Reister

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