Drop support for < CMake 3.10?

CMake is now warning if you have `cmake_minimum_required(VERSION xxx)` less than 3.10. This makes our builds very noisy as hunter uses 3.5 a lot.

Could either make minimum policy 3.12 but still support 3.5

```
cmake_minimum_required(VERSION 3.5...3.12)
```

OR

lose support for < 3.10 entirely?

```
cmake_minimum_required(VERSION 3.10)
```