From source installation of Gazebo Classic does not cover the Ubuntu 22.04

[traversaro]

The "installation from source" tutorial of Gazebo Classic, available at:

• https://github.com/osrf/gazebo_tutorials/blob/95500a516859f5b604d694bbe316a30e0e2d8b8a/install_from_source/tutorial_default.md?plain=1
• http://classic.gazebosim.org/tutorials?tut=install_from_source&cat=install

is confusing, as it does refer to Ubuntu 22.04 (see https://discordapp.com/channels/1077825543698927656/1080763334384697430/1184131057038147615).

In particular, that tutorial does not work out of the box on Ubuntu 22.04, as it assumes that all the dependencies of Gazebo are installed in binary form from OSRF repos. However, Gazebo Classic 11/Ignition Citadel binaries are not available for Ubuntu 22.04, so the possible options are:

• Either mention in the docs that Ubuntu 22.04 is not supported
• or document the installation from source of all the packages listed in https://github.com/gazebo-tooling/gazebodistro/blob/master/gazebo11.yaml, probably using colcon as documented in https://colcon.readthedocs.io/en/released/user/quick-start.html#build-gazebo-and-the-ignition-packages