deps: colorstring library is archived

[jrasell]

The library we use for CLI coloring has not been committed to for 7 years and is archived: https://github.com/mitchellh/colorstring

We should look into migrating from this library to something that is maintained. fatih/color could be a candidate.

[tgross]

Having used both libraries in other projects, the colorstring library has the advantage that you can build up a string that has differently-colored segments using strings.Builder or concatenation and send that string to a writer later. As opposed to in fatih/color, which is a perfectly fine library and similar to others, where you have to build the string up with method calls. So the DX of colorstring is a little more "magic" here and makes for a better DX in use cases where performance isn't critical.

It also seems like colorstring is just "done", rather than archived because it shouldn't be used. If it's annoying a vulnerability scanner, we should probably just vendor the bits of it we use in Nomad; it's less than 150 lines of non-comment code and half of that is the color table.