Fix license in metadata.desktop files
Summary:
The license key is specified as GPL, while the actual code (I checked)
in all these cases is licensed under GPL version 2 or later. This commit
fixes the metadata.desktop files to reflect the actual license.
Test Plan: Manually checked, also compared to the keys looked for in kaboutdata.cpp
Reviewers: Plasma, sitter, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D6627