Doesn't build on Ruby 4.1 because untyped data API got removed

I'm testing against ruby-head and rugged doesn't build against Ruby 4.1 anymore. Ruby removed some deprecated functions recently (ruby/ruby#15447), which caused this.

I found #998 which already moved most things over but it looks like the comment about the backend has now become relevant (: There's also one gem-controlled instance here that needs to be changed