More license maintenance: GPLv2 -> GPLv2+

Summary:
This should now cover all remaining GPLv2-only files in messagelib,
excluding test code. This is largely based on existing approval from
https://techbase.kde.org/Projects/KDE_Relicensing.

Missing are KDAB (hereby approved) and Andras (hopefully approving by
+2'ing this review).

Reviewers: amantia

Reviewed By: amantia

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D16000