chore: Promote @rimolive as a backend approver

[chensun]

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[google-oss-prow]

@chensun: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kubeflow-pipeline-upgrade-test	bdc1a65	link	false	/test kubeflow-pipeline-upgrade-test
kubeflow-pipeline-e2e-test	bdc1a65	link	false	/test kubeflow-pipeline-e2e-test

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[rimolive]

/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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• backend/OWNERS

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