Use btree from k8s.io/utils instead of github.com/google/btree

The google/btree package is deprecated, so switch to the maintained
fork in k8s.io/utils/third_party/forked/golang/btree.

API differences:
- NewG -> New
- BTreeG[T] -> BTree[T]

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

Kubernetes-commit: a328ca88ad662119bbf129cd1107fabd8dd9f539