Use the SPDX license identifier in appdata XML
The Flatpak build - and possibly other packaging methods - require the license to be a valid identifier according to the SPDX list, so set it to GPL-3.0-or-later
as specified on there.
The Flatpak build - and possibly other packaging methods - require the license to be a valid identifier according to the SPDX list, so set it to GPL-3.0-or-later
as specified on there.