[Development] Proposal to raise the minimum CMake version needed to build and use Qt 6.9

[Tim Blechmann]

> We'd like to bump the minimum required CMake version for building and using Qt to CMake 3.26.
> 
> The pending change is https://codereview.qt-project.org/c/qt/qtbase/+/603886
> 
> The rationale is easier maintenance of build system code, and allowing to provide better public cmake apis and behaviors
> (cleaner qml module apis, better deployment handling, better android handling, better static linking handling, to name a few).
> 
> The timing is good because Qt 6.9 is non-LTS.

one point to consider is that cmake does not have any run-time 
dependencies and it is extremely easy to install independently from the 
OS: the binaries provided by kitware are fully self-contained and can be 
installed in parallel to system-provided cmake.
if it eases the maintenance and allows better public APIs, it does not 
seem unreasonable to bump the minimum required version.

tim