Analitza6Config: Add Qt6Core5Compat to dependencies

It is part of the public link interface, so cmake should make
sure it is available.
I suppose its absence from the cmake config file, is the reason it
was added to kalgebra [1], which doesn't really seem to use it
directly itself.
While at it, also use the right variable with the minimum Qt version.

[1] kalgebra@137d730f

(cherry picked from commit bdbec190)