Add a version constraint to exclude KF6-alpha releases on v5 branch

245.0 is supposed to be the start of alpha/beta releases of KF6 components. Anything less than that is assumed to be a stable release of KF5.

alpha, beta, rc, stable of KF6 will go as v5.245.0, v5.246.0, v5.247.0, ...., v6.0.0

The constraint needs to change again when KF6 -> KF7 happens, assuming KF7 starts somewhere in the v6 branch, at v6.249.0 (say)

(Needs backport to qt5.15lts-22.08 as well)