Add UsageFunc to deprecation section in 3.7 changelog.

Signed-off-by: Ahmad Zolfaghari <ah8ad3@gmail.com>

CHANGELOG/CHANGELOG-3.7.md

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+Previous change logs can be found at [CHANGELOG-3.6](https://github.com/etcd-io/etcd/blob/main/CHANGELOG/CHANGELOG-3.6.md).
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+<hr>
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+## v3.7.0 (TBD)
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+### Deprecations
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+- Deprecated [UsageFunc in pkg/cobrautl](https://github.com/etcd-io/etcd/pull/18356).