[startplasma] don't set QT_AUTO_SCREEN_SCALE_FACTOR with Qt >= 5.14

Summary:
It gives the following runtime warning:
   Warning: QT_AUTO_SCREEN_SCALE_FACTOR is deprecated. Instead use:
      QT_ENABLE_HIGHDPI_SCALING to enable platform plugin controlled per-screen factors

But we don't need to set QT_ENABLE_HIGHDPI_SCALING to 0.
Just setting QT_SCREEN_SCALE_FACTORS is enough to control scaling, after
my commit e018d11600bffc6 to qtbase.

Kudos for porting startkde to be C++ code, it makes this commit much
easier...

Test Plan: Builds; not rebooted yet

Reviewers: apol, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24255

startkde/startplasma.cpp

@@ -218,9 +218,11 @@ void runEnvironmentScripts()
 
 void setupPlasmaEnvironment()
 {
+#if QT_VERSION < QT_VERSION_CHECK(5, 14, 0)
     //Manually disable auto scaling because we are scaling above
     //otherwise apps that manually opt in for high DPI get auto scaled by the developer AND manually scaled by us
     qputenv("QT_AUTO_SCREEN_SCALE_FACTOR", "0");
+#endif
 
     qputenv("KDE_FULL_SESSION", "true");
     qputenv("KDE_SESSION_VERSION", "5");