Refactor gripper move groups to use chains instead of links.

[Moth-Man]

Closes #8102

Simplify planning groups in config SRDFs to use chains as opposed to list of links.

[nbbrooks]

Could you also update

• https://github.com/PickNikRobotics/moveit_studio_ur_ws/blob/main/src/picknik_ur_multi_arm_config/config/moveit/multi_arm_ur.srdf
• https://github.com/PickNikRobotics/moveit_studio_ur_ws/blob/main/src/picknik_ur_gazebo_config/config/moveit/picknik_ur_gazebo.srdf.xacro

[nbbrooks]

I verified that picknik_ur_multi_arm_config will launch and trajectory based motions succeed. Something is misconfigured with servo though which results in servo based Teleop modes not working, but that is unrelated to this change.