Migrate away from setup.rb

[henk84]

setup.rb is some project that has been abandoned over a decade ago:

• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=897308
• https://i.loveruby.net/en/projects/setup/doc/usage.html
• https://directory.fsf.org/wiki/Setup.rb

The gem that was considered a replacement also seems abandoned, last update 2012:

• https://rubygems.org/gems/setup/versions/5.2.0

Modern way seems to be gem and/or bundle.
We should look into using that instead.

Starting points:

• https://guides.rubygems.org/make-your-own-gem/
• https://bundler.io/guides/creating_gem.html
• https://wiki.debian.org/Teams/Ruby/Packaging (IMHO to consider to make sure the chosen approach is also convenient for packaging for linux distributions)

Any further inputs are welcome!