Use cmake_minimum_required instead of cmake_policy

[LecrisUT]

Syntax like cmake_minimum_required(VERSION 3.20...3.29) sets policies for up to 3.29 automatically. And since a new policy stack is created even when using include and find_package, all of these should be safe ¹

Footnotes

1. https://cmake.org/cmake/help/latest/command/cmake_policy.html#cmake-policy-stack ↩