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/applications/audio/sonic-pi/default.nix

@@ -105,5 +105,7 @@ mkDerivation rec {
     license = lib.licenses.mit;
     maintainers = with lib.maintainers; [ Phlogistique kamilchm ];
     platforms = lib.platforms.linux;
+    # sonic-pi depends on ruby 2.4 which we don't support anymore
+    broken = true;
   };
 }