Add classic name to markdown references to gazebo

[Ryanf55]

Purpose

Make it clear that this is for the older gazebo (Gazebo Classic).

Details

I split this into two commits. The first updates the main README, while the 2nd does more of a find-replace. I only changed markdown files. I did my best to leave out any code blocks from the changes, and tried to change the name of "Gazebo Answers" to "Gazebo-classic" answers. It took about 30 minutes to go through the diff.

Perhaps the PR should be split to merge the first commit, and then discuss the 2nd in more detail on if this is a good idea?

Issue

Closes #177