Fix MotionPlanning combo box signals for Qt6

[andrearosasco]

Summary

Fix the MotionPlanning RViz plugin's named-state combo box connections when built against Qt6.

Qt6 no longer exposes the old text-carrying QComboBox signal names used by the plugin's string-based connections:

• activated(QString)
• currentIndexChanged(QString)

In a Qt6 RViz environment this produces runtime warnings such as:

QObject::connect: No such signal QComboBox::activated(QString)
QObject::connect: No such signal QComboBox::currentIndexChanged(QString)

When those connections fail, selecting a named Start State or Goal State in the MotionPlanning panel does not invoke the corresponding handler, so selecting a named goal such as home does not update the query goal state.

For more details check #3744

Changes

• Use QComboBox::textActivated for Start State and Goal State under Qt6.
• Use QComboBox::currentTextChanged for Planning Group under Qt6.
• Keep the existing Qt5 string-signal connections for Qt5 builds.

Testing

Tested downstream in a RoboStack Lyrical Qt6 RViz environment by rebuilding moveit_ros_visualization with this patch and verifying that the MotionPlanning panel no longer logs the missing QComboBox signal warnings and named goal-state selection works again.