Build packages for both Gazebo Harmonic (gz-sim8) and Gazebo Ionic (gz-sim9)

[traversaro]

We currently build packages just for Harmonic (gz-sim8), but thanks to robotology/gz-sim-yarp-plugins#208 we support also Gazebo Ionic (gz-sim9). We should build packages for Gazebo Ionic also in here. One thing we need to decide is to which package give more priority, perhaps it make sense to have higher priority for gz-sim9 packages.