KDE: Update KDE Plasma to 6.7.3

Plasma 6.7 is the last release supporting X11 session. Future KDE Plasma 6.8
release will be Wayland-exclusive. Support for X11 applications will be fully
entrusted to Xwayland. For more info. see KDE blog post:
https://blogs.kde.org/2025/11/26/going-all-in-on-a-wayland-future/

Announcements:
https://kde.org/announcements/plasma/6/6.7.0
https://kde.org/announcements/plasma/6/6.7.1
https://kde.org/announcements/plasma/6/6.7.2
https://kde.org/announcements/plasma/6/6.7.3

Ports changes:

x11-themes/plasma6-union: Add new Plasma port.
Plasma Union is Qt style engine supporting both QtQuick and QtWidgets.

Mk/Uses/kde.mk:

• Add union component
• fix path for print-manager component

deskutils/plasma6-xdg-desktop-portal-kde:

• Drop unused KDE components

graphics/plasma6-spectacle:

• Require graphics/tesseract

print/plasma6-print-manager

• Depend on packagekit-qt
• Renew KDE components

sysutils/plasma6-ksystemstats:

• Regenerate patch

x11-toolkits/plasma6-kdeplasma-addons

• Require devel/corrosion for building
• Renew USE_KDE and USE_QT components

x11-wm/plasma6-kwin

• Require Vulkan
• Renew KDE components

x11/plasma6-kscreen

• Renew KDE components

x11/plasma6-plasma-desktop:

• Drop unused KDE components

x11/plasma6-plasma-integration:

• Drop unused KDE components