Use KNotifications 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 KNOTIFICATIONS_BUILD_DEPRECATED_SINCE(5, 79) returning true again with KF6.