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

[Earlopain]

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

rugged/ext/rugged/rugged.c

Line 690 in 0772a8e

rb_mShutdownHook = Data_Wrap_Struct(rb_cObject, NULL, &cleanup_cb, NULL);