Request the GuiPrivate component only on Qt6

Listing GuiPrivate unconditionally in QET_COMPONENTS breaks the whole
Qt5 configure: find_package(Qt5 COMPONENTS GuiPrivate) looks for a
Qt5GuiPrivateConfig.cmake that has never existed - Qt5 creates the
Qt5::GuiPrivate target implicitly together with Gui. Only Qt6 requires
(and provides) the explicit component.

Move the request into a QT_VERSION_MAJOR-guarded find_package after the
main one, both for the application and for tests/catch (whose targets
link Qt::GuiPrivate via QET_PRIVATE_LIBRARIES). Fixes the msys2/Qt5
Windows CI configure failure:
"Could not find a package configuration file provided by Qt5GuiPrivate".

Verified: Qt 6.11 configure passes and the Qt6::GuiPrivate target is
created (the private-module warning now fires from the guarded call).
The Qt5 path simply no longer requests the component, restoring the
pre-existing implicit behaviour.

tests/catch/CMakeLists.txt

@@ -59,6 +59,13 @@ find_package(
  ${QET_COMPONENTS}
  REQUIRED)
 
+# Qt6-only: the tests link ${QET_PRIVATE_LIBRARIES}, which contains
+# Qt::GuiPrivate; Qt6 only creates that target when the component is
+# explicitly requested (Qt5 creates it implicitly and has no such package).
+if(QT_VERSION_MAJOR GREATER_EQUAL 6)
+  find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS GuiPrivate)
+endif()
+
 Include(FetchContent)
 
 FetchContent_Declare(