ruby_2_4: remove

According to https://endoflife.software/programming-languages/server-side-scripting/ruby
ruby 2.4 will go end-of-life in march, where the new release of nixpkgs
will be cut. We won't be able to support it for security updates.

Remove all references to ruby_2_4 and add ruby_2_7 instead where
missing.

Mark packages that depend on ruby 2.4 as broken:
* chefdk
* sonic-pi