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/Common/LtsVirtualKeyboard/LtsVirtualKeyboard.cpp

@@ -3,18 +3,18 @@
 #include "LtsVirtualKeyboard.h"
 
 #include <LtsAppManager/lts_app_manager.h>
-#include <QApplication>
-#include <QClipboard>
+#include <QtWidgets/QApplication>
+#include <QtGui/QClipboard>
 #include <QDoubleSpinBox>
-#include <QHBoxLayout>
-#include <QKeyEvent>
-#include <QLineEdit>
-#include <QPlainTextEdit>
-#include <QScreen>
-#include <QScrollArea>
-#include <QSpinBox>
-#include <QTextCursor>
-#include <QTimer>
+#include <QtWidgets/QHBoxLayout>
+#include <QtGui/QKeyEvent>
+#include <QtWidgets/QLineEdit>
+#include <QtWidgets/QPlainTextEdit>
+#include <QtGui/QScreen>
+#include <QtWidgets/QScrollArea>
+#include <QtWidgets/QSpinBox>
+#include <QtGui/QTextCursor>
+#include <QtCore/QTimer>
 #include <QWindow>
 
 namespace LtsVirtualKeyboard