Qt6 migration: Fix BUILD_RTI=OFF build errors in TrainingAreaBase

- training_area_telegrams.cpp: Fixed QJsonObject initializer syntax (Qt6 requires {{}} not ({}))
- training_area_telegrams.cpp: Fixed fromJson() methods to access m_* members directly
- training_area_telegrams.cpp: Fixed brace mismatches in multiple toJson() methods
- training_area_telegrams.h: Changed Header struct to public members for consistency
- tx_mixin.cpp: Fixed getEquipmentNumber() template for direct member access
- lts_coordinate.cpp: Added missing #include <iomanip>
- MIGRATION_STATUS.md: Added migration progress tracking document

C++17 constraint enforced due to RTI Connext DDS 7.7.0 incompatibility with C++20+

Build status: BUILD_RTI=OFF successful (0 errors, warnings only)

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

qt6-after/qt6-migrated-legatus/Lts/Plugins/VirtualKeyboard/Gui/VirtualKeyboard/vkb_gui_plugin.cpp

@@ -9,15 +9,15 @@
 #include <LtsVirtualKeyboard/LtsVirtualKeyboard.h>
 #include <LtsVirtualKeyboard/LtsKeyPushButton.h>
 
-#include <QApplication>
+#include <QtWidgets/QApplication>
 #include <QDesktopWidget>
 #include <QDoubleSpinBox>
-#include <QLineEdit>
-#include <QPlainTextEdit>
-#include <QScrollArea>
-#include <QSpinBox>
-#include <QTextEdit>
-#include <QComboBox>
+#include <QtWidgets/QLineEdit>
+#include <QtWidgets/QPlainTextEdit>
+#include <QtWidgets/QScrollArea>
+#include <QtWidgets/QSpinBox>
+#include <QtWidgets/QTextEdit>
+#include <QtWidgets/QComboBox>
 #ifdef Q_OS_WIN
 #include <windows.h>
 #endif

qt6-after/qt6-migrated-legatus/Lts/Plugins/VirtualKeyboard/Gui/VirtualKeyboard/vkb_gui_plugin.h

@@ -6,7 +6,7 @@
 
 #include <Plugins/VirtualKeyboard/vkb_gui_plugin_interface.h>
 #include <NGLLogger/INGLLogger.h>
-#include <QPointer>
+#include <QtCore/QPointer>
 
 namespace LtsVirtualKeyboard
 {