Consistent CMake requirements in Plasma

[Nicolas Fella]

Hi,

while I was doing some cmake cleanup in Plasma I noticed that our stated
minimum cmake versions are both somewhat inconsistent and super old.
Most Plasma projects state that either 2.8 (released in 2009) or 3.0
(released in 2014) are the minimum. That's obviously unrealistic.

Newer cmake versions offer some nice stuff such as imported targets for
common libs (e.g. used in
https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/336). For
the sake of consistency I propose that we standardize Plasma on
something recent-ish.

My candidate for this would be cmake 3.16. It's the version shipped by
Ubuntu 20.04 and thus Neon. This would exclude Debian stable which ships
3.13 (which does not have the feature relevant for
https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/336).

Thoughts?

Cheers

Nico