License maintenance: GPLv2 -> GPLv2+

Summary:
This is needed to ensure compatibility with Apache 2 licensed code
(e.g. zxing, future OpenSSL).

This is largely based on existing approval from contributors for such
changes as listed on https://techbase.kde.org/Projects/KDE_Relicensing.
Missing are KDAB (hereby approved) and Andras (hopefully approved by
+2'ing this review).

Reviewers: amantia

Reviewed By: amantia

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D15981