ros2_tools_resources.md: update for Foxy

[gerkey]

From #15:

• The main change (to be resolved later, after the merge, in a different PR, no need to hold it up now) is that we'll need the instructions to refer to ROS 2 Foxy because of other dependencies that require us to use Ubuntu 20.04, which means ROS 2 Foxy.
• Since Eloquent will be EOL by the end of the year, I suggest that we start with Foxy in this sentence, and update any textual differences as needed throughout (later) after the chapter is merged. We actually need the gcc version in Ubuntu 20.04 for the very latest in rmf_core, which means we'll need this initial release to be on Foxy. Perhaps this paragraph can be just slightly adjusted (for now) to say that although the current version of this chapter refers to Eloquent throughout, it will soon be updated to refer to Foxy. It seems virtually all the content will be identical.

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