Creating a KDE Frameworks minimum cmake version policy

[Albert Astals Cid]

I would like to propose this minimum cmake version policy for KDE Frameworks.

The required minimum cmake version for KDE Frameworks will be the maximum of:
 * The required cmake version of required Qt at that time
 * The cmake version released 1 year before the required Qt at that time

Let's do the calculations for now.

required Qt at the time = 6.8
required cmake version of Qt 6.8 = 3.16
Qt 6.8 release date = 8 October 2024 
1 year before Qt 6.8 relase date = 8 October 2023

cmake 3.27.0 release date = 18 July 2023
cmake 3.28.0 release date =  6 December 2023

cmake version released 1 year before the required Qt at that time = 3.27

So this policy would suggest to increase our minimum cmake requirement to 
max(3.27, 3.16) -> 3.27

This ties updating the minimum cmake version to when we update Qt which i 
think makes sense, if we are going to as people to update Qt, we may as well 
ask them to update cmake (which is in my opinion much easier)

I know it is quite a jump in minimum required cmake version but i think having 
a policy is much simpler than having to justify every time we want to do an 
update.

What do you all think? 

Cheers,
  Albert

P.S: https://www.wikidata.org/wiki/Q1023904is useful to see the release dates 
of cmake