chore: Adding @HumairAK to backend and frontend reviewers

[chensun]

Description of your changes:
also cleanup inactive owners

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow]

@chensun: The following test 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	bfef49e	link	false	/test kubeflow-pipeline-upgrade-test

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[terrytangyuan]

Thanks!

/lgtm

[jlyaoyuli]

/lgtm
/approve

[chensun]

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun, jlyaoyuli, terrytangyuan

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• backend/OWNERS [chensun]
• frontend/OWNERS [chensun,jlyaoyuli]

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