Migrate google/btree usage to k8s util package

[pjsharath28]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

The github.com/google/btree library used by k8s and etcd has been archived and is no longer maintained. This raises long-term concerns around security fixes and CVEs.

We discussed multiple options to address this in the issue. Given that we only rely on a single source file from the library, we concluded that copying the code into k8s.io/utils is the simplest option. This avoids introducing or maintaining a separate fork while preserving identical behaviour.

Which issue(s) this PR is related to:

Discussion issue: etcd-io/etcd#20991

Special notes for your reviewer:

Upstream Google attribution is preserved and the file header follows Kubernetes conventions while remaining Apache-2.0 compliant.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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This pull-request has been approved by: pjsharath28
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[aojea]

based on the linked discussion I think @dims referred to https://github.com/kubernetes/utils

[pjsharath28]

Sure, thanks for the clarification @aojea

I have raised the PR in utils repo kubernetes/utils#336 for the same and hence closing this PR.