x11/plasma5-plasma-workspace: another upstream backport

From upstream
https://invent.kde.org/plasma/plasma-workspace/-/commit/f77b7284e39b14b0e9a8b4d2b77c0b93b2c9ea59

Before this commit ThemesModel only finds metadata.desktop, but after
KF5.94, the default theme metadata files have been ported to json format.

BUG: 453830