Some developers have multiple versions of Qt installed on their system.
And source code often has some version-dependent code paths, e.g. using
newer API if found available. In that case the resulting build has an
effective dependency on the Qt version built against.
To properly reflect that, the Qt version set as minimally required version
of the listed dependencies in the generated CMake config files should use
that version instead.
While the current code of KCoreAddons does not have such version dependent
code variants, they are added now and then, so using the Qt version built
against as minimally required dependency is a good default, and should not
change anything for all the users with just one Qt version, and improve
things for those with, catching bad mixes early in the build time.