Bump tested CMake version to 3.18

[tambry]

Use the version range feature introduced in 3.12. On CMake <3.12 the extra dots are simply interpreted as extra version number separators.
The fallback for ancient CMake versions is kept.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. Is there any benefit to using a version range?

[tambry]

Thanks for the PR. Is there any benefit to using a version range?

Compared to the current approach of an if it's simpler.

Otherwise, using a range enables policies for versions up to the max version (usually the latest tested version).
So with cmake_minimum_required(VERSION 3.1...3.18) CMake 3.1 is the minimum version required. Using CMake 3.14 you get the NEW policies up to version 3.14. Using CMake 3.20 you'd get policies up to version 3.18.