Update ros_ign to ros_gz in Rolling

[azeey]

See ros2/variants/pull/42

[sloretz]

• https://index.ros.org/p/ros_gz_bridge/
• https://index.ros.org/p/ros_gz_sim/
• https://index.ros.org/p/ros_gz_image/
• https://index.ros.org/p/ros_gz_interfaces/
• https://index.ros.org/p/ros_gz_sim_demos/

[clalancette]

I think we are waiting for ros/sdformat_urdf#28 and gazebosim/ros_gz#531 to land here. Once those are in, the packages can actually build on the buildfarm and then the simulation variant should build. I think we should probably wait to merge this until that happens.