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