Lower CMake version requirement
Lowered the minimal required CMake version to 3.15. This requirement is also fulfilled when using Ubuntu 20.04 LTS, and it paves the way for using scikit-build-core in the future, which requires CMake >= 3.15.
Lowered the minimal required CMake version to 3.15. This requirement is also fulfilled when using Ubuntu 20.04 LTS, and it paves the way for using scikit-build-core in the future, which requires CMake >= 3.15.