Add Humble CI

[chapulina]

The galactic branch supports versions from Galactic and higher, as described on the changelog. CI only runs for Fortress though.

This PR:

• Moves Rolling CI to Jammy (22.04)
• Adds the use of the testing repo, which is good practice in CI so we get packages that have been bloomed, but not synced yet
• Also test the Fortress + Humble combination

The action is currently failing with Failed to detect successful installation of [ros-rolling-gazebo-ros-pkgs] because of

• https://github.com/ros-simulation/gazebo_ros_pkgs/issues/1359