[cmake] Require CMake 3.15 or newer

[gruenich]

• The currently used version of Google-test 1.13 still triggers a CMake deprecation warning with CMake 3.30 or newer. We need to decide on which version of Google-test we want to update. I see no reason to not choose the most recent version 1.16 which requires C++14 (like Google-test 1.13) and CMake 3.13. (I went for Google-test 1.16)
• Adjust CI to test for both CMake 3.15 and 4.0, the new min and max required version.

[henryiii]

For context, googletest does not specify a range, which is why the minimum matters there.

[gruenich]

@henryiii How did you image testing with distinct versions of CMake? Downloading and building CMake from source seems excessive.

[henryiii]

I do it in pybind11 and CLI11 (local support action). Basically jwlawson/actions-setup-cmake downloads binaries pretty quickly.