[applets/systemtray] Downgrade required qtquick

Next Plasma has a Qt 5.12 dependency

applets/systemtray/package/contents/ui/ExpandedRepresentation.qml

@@ -17,8 +17,8 @@
  *   51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  */
 
-import QtQuick 2.13
-import QtQuick.Layouts 1.13
+import QtQuick 2.12
+import QtQuick.Layouts 1.12
 
 import org.kde.plasma.core 2.0 as PlasmaCore
 import org.kde.plasma.components 3.0 as PlasmaComponents