[Development] Raising minimum CMake version to 3.16 for Qt6

[Kyle Edwards]

On 5/5/21 1:58 PM, Thiago Macieira wrote:
> 3.16.0 was tagged on Nov 2019, which is a little too close for comfort. For
> comparison, we require GCC 8, which was released in May 2018.
>
> However, given that no one has apparently complained and that upgrading cmake
> is far easier than the compiler, I have no objections.

CMake upstream here. We recommend always using the very latest patch 
version of CMake, since it is backwards compatible and can 
(theoretically, bugs notwithstanding) build CMake projects written for 
any earlier version of CMake. You are correct that upgrading CMake is 
generally easy, though it admittedly may not be feasible if you insist 
on sticking with the version provided by the distro (though we do 
provide Ubuntu .deb builds of the latest version.)

Kyle