In order to migrate to Qt6 all options for KF6 were added:

a) using a system provided KF6
b) downloading and compiling KF6
c) using the vendored-in re-creation of the functionality

The behaviour for both Qt5 and Qt6 is steered with the same two variables which were renamed to become version agnostic:
a) BUILD_WITH_KF=ON BUILD_KF=OFF
b) BUILD_WITH_KF=ON BUILD_KF=ON
c) BUILD_WITH_KF=OFF

The version is automatically derived from the chosen Qt major version.

sources/editor/ui/dynamictextfieldeditor.h

@@ -21,7 +21,7 @@
 #include "../elementitemeditor.h"
 #include "../graphicspart/partdynamictextfield.h"
 
-#ifdef BUILD_WITHOUT_KF5
+#ifdef BUILD_WITHOUT_KF
 #else
 #	include <KColorButton>
 #endif
@@ -79,7 +79,7 @@ class DynamicTextFieldEditor : public ElementItemEditor {
 		QList<PartDynamicTextField*> m_parts;
 		QList<QMetaObject::Connection> m_connection_list;
 
-	#ifdef BUILD_WITHOUT_KF5
+	#ifdef BUILD_WITHOUT_KF
 	#else
 		KColorButton* m_color_kpb = nullptr;
 	#endif