Bump `cmake_minimum_required(VERSION 3.15)`

There isn't really a good reason to use 3.15 and people appear to just copy things from the examples (relevant discussion).

It is always easy to get the latest CMake version in your CI, so probably best to just bump it: https://alexreinking.com/blog/how-to-use-cmake-without-the-agonizing-pain-part-1.html