mission_raw: disable mission_changed in pure mode

src/mavsdk/plugins/mission_raw/mission_raw_impl.cpp

@@ -517,6 +517,13 @@ MissionRaw::MissionProgress MissionRawImpl::mission_progress()
 MissionRaw::MissionChangedHandle
 MissionRawImpl::subscribe_mission_changed(const MissionRaw::MissionChangedCallback& callback)
 {
+    // Listening for MISSION_ACKs can work, however, it is not guaranteed that these acks
+    // will be received. This should be specced properly, hence it's not in Pure mode.
+    if (_parent->compatibility_mode() == System::CompatibilityMode::Pure) {
+        LogErr() << "mission changed subscription not supported";
+        return;
+    }
+
     std::lock_guard<std::mutex> lock(_mission_changed.mutex);
     return _mission_changed.callbacks.subscribe(callback);
 }