Use QT_MIN_VERSION & KF5_MIN_VERSION variables

Start off with versions derived from old "find_package(ECM 5.30)",
so KF5 5.30 and its minimum Qt dependency version 5.5,
as there are some external non-Plasma consumers of this library
(like KDevelop) which have more relaxed minimal version dependencies
and which still want to be able to build with recent libksysguard
against the same older dependencies (like on KDE CI)