Make asdf the default package manager

asdf can be used to install all prerequisites (according to @gtsiolis 😸). So for many people, who aren't passionate about any of the other suggested tools (rbenv, chruby, etc), we can recommend to install prerequisites with asdf.

Having one tool, can reduce the number of steps, it makes it more consistent across platforms: e.g. no more golang through homebrew on macOS vs some ppa on Ubuntu.

Any other installation method we have now (rvm, rbenv, ...) are supported in parallel.