SubTreePlus nodes should use __autoremap to access blackboard items used in nav2

In versions of ros-humble-behaviortree-cpp-v3 before v3.8.4, SubTree nodes could access parent's blackboard entries as explained here: https://bugs.eclipse.org/bugs/show_bug.cgi?id=581733.

From v3.8.4 only the use of __autoremap=true allows automatic remapping in a clean way: https://github.com/ros-navigation/navigation2/issues/3640#issue-1765269417

The BT XML exporter in P4R must be updated accordingly.