Recommend using gazebo's rosdep sources to satisfy package.xml dependencies

[Ryanf55]

Context:

• ROS: update rosdep command ardupilot_wiki#6359 (comment)

• ardupilot_ros/.github/workflows/colcon.yml

  Line 44 in 940efb9

  wget https://raw.githubusercontent.com/osrf/osrf-rosdep/master/gz/00-gazebo.list -O /etc/ros/rosdep/sources.list.d/00-gazebo.list

Can we recommend to users to use the gz rosdep sources when they install the ardupilot_ros toolchain?

As of the linked wiki PR, we now recommend -r which ignores errors. It's a support burden because many people ignore errors and file support when their stuff doesn't build because they skipped steps.

I was hoping our new contributor @sauk2 could take this on, which is largely a documentation activity.