x11/plasma5-plasma-workspace: add upstream bug fix

applets/appmenu: fix top-level menu text coloration

Commit [1] made the color dynamic, but neglected to respect the
Plasma color scheme, if any. This causes problems with Plasma
themes that have their own colors and don't respect the
systemwide color scheme, such as Breeze Twilight.

Fix it by using the appropriate colors from the PlasmaCore color
scheme object, not the systemwide object provided by Qt.

[1] https://invent.kde.org/plasma/plasma-workspace/-/commit/28537cf3ff3cd9210f7568f40334ac3a2c9bed18

Obtained from: https://invent.kde.org/plasma/plasma-workspace/-/commit/4d3f99558cff95259590e70dfbf854a479f772ce
Upstream PR: https://bugs.kde.org/show_bug.cgi?id=453348