Use KF5_MIN_VERSION instead of KF5_VERSION

Summary:
KF5_VERSION is overridden by first KF5 find_package version found result, which may be higher
than other installed Frameworks versions at the time, leading to cmake error.

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D17684