Fix compilation against Qt6 and add CI

src/models/completionmodel.h

@@ -3,10 +3,9 @@
 
 #pragma once
 
+#include <QConcatenateTablesProxyModel>
 #include <QSortFilterProxyModel>
 
-#include <KConcatenateRowsProxyModel>
-
 #include "roomlistmodel.h"
 
 class CompletionProxyModel;
@@ -75,6 +74,6 @@ private:
 
     UserListModel *m_userListModel;
     RoomListModel *m_roomListModel;
-    KConcatenateRowsProxyModel *m_emojiModel;
+    QConcatenateTablesProxyModel *m_emojiModel;
 };
 Q_DECLARE_METATYPE(CompletionModel::AutoCompletionType);