Use all-upper-case version variable names (KF consistency), also K-prefix

All (but 3 currently) KF modules use UPPERCASE_VERSION pattern for the version macros/variables. So let's be more consistent to allow more writing-by-patters :)

No plan to backport to KF5, as this is a SC-breaking change. IMHO anyone who might want to have code both compatible with KF5 & KF6 would need to do local support for both variants where needed.

And as this macro is used in preprocessor condition usually, at least I know of no way to add deprecation warnings, so preparing things on the KF5 side might not be possible.

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