License maintenance: GPLv2 -> GPLv2+

Summary:
For most authors we have approval for such changes listed on
https://techbase.kde.org/Projects/KDE_Relicensing, explicit
approval is only needed from KDAB (hereby granted) and André
Heinecke.

Reviewers: aheinecke

Reviewed By: aheinecke

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D16322