Use KF/KIO version as condition for backward compat-support

Builds of KF modules with certain specific levels of excluded
deprecated API and its implementation never got full support (e.g.
by CI) and care by developers, only worked by chance.
So the feature is not really reliable, only ever was used as rough
KF6 preparation help. Thus nothing that needs support here.

Switching instead to check for some pre-KF6 version achieves the same
effect in current CI and developer setups, as well as also deals with
KIOCORE_BUILD_DEPRECATED_SINCE(5, 69) returning true again with KF6.