Require KF 5.85 and enforce more strict compiler settings
Also switch to use KDECompilerSettings and default to min required ECM version as KDE_COMPILERSETTINGS_LEVEL value
This should give an example how to switch to KDECompilerSettings, and is what needs to be done in all other cases as well.
I understand it is a bit annoying to have to change more than just bumping the version now. But the reppurposing of KDEFrameworksCompilerSettings was sadly a shortcut at the beginning which at one point in time has to be resolved in favour of an officially supported way to get strict settings. I am happy to help out where I can here.