Revert the KF5 version bump

It broke the build due to use of APIs that are deprecated in the new
KF_DISABLE_DEPRECATED_BEFORE_AND_AT setting.

This reverts commit ea46b509.
2 jobs for master in 22 minutes and 29 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Build
passed build_clazy_clang_tidy #105591

00:13:37

passed linux kf5-qt5 qt5.15 #105590

00:08:51

10.0%