Fix Qt5:: target references and add missing participant_info header

- Replace all Qt5:: target references with Qt6:: in 26 CMakeLists.txt files
- Add missing participant_info_gui_element_interface.h to ParticipantManagement plugin

Qt6 migration: All CMake target_link_libraries now use Qt6::Core, Qt6::Widgets,
Qt6::Network instead of Qt5:: equivalents.

Validated with BUILD_RTI=ON and BUILD_RTI=OFF - both builds successful.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/Lts/Plugins/ParticipantManagement/participant_info_gui_element_interface.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <QtPlugin>
+#include <QWidget>
+
+namespace LtsSession
+{
+class LtsSession;
+}
+
+class GuiElementInterface
+{
+public:
+  GuiElementInterface() = default;
+  virtual ~GuiElementInterface() = default;
+  virtual QString pluginName() = 0;
+  virtual QWidget* createGuiElement(QWidget *parent = nullptr) = 0;
+  virtual bool init(LtsSession::LtsSession *session, int participantId = 0) = 0;
+};
+
+// Eindeutige ID für das Qt-Plugin-System
+#define IGuiElementPlugin "gui_element:3:participantInfo.guiElement"
+Q_DECLARE_INTERFACE(GuiElementInterface, IGuiElementPlugin)