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

pkgs/development/ruby-modules/with-packages/test.nix

@@ -6,7 +6,6 @@ let
   stdenv = pkgs.stdenv;
 
   rubyVersions = with pkgs; [
-    ruby_2_4
     ruby_2_5
     ruby_2_6
     ruby_2_7