Grant maintainers-etcd admin permission on etcd-io/etcd

[jmhbnz]

To edit an incorrect security advisory repo admin permission is required. Refer:

• https://docs.github.com/en/code-security/security-advisories/working-with-repository-security-advisories/editing-a-repository-security-advisory
• https://docs.google.com/document/d/1Pl2e8YTtgt3pYeIZSXJIn4gknp6tTW14htMat1JaM4k
• GHSA-5x4g-q5rc-36jp

This commit will be reverted once the remedial work is done.

[dims]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims, jmhbnz

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• config/etcd-io/sig-etcd/OWNERS [jmhbnz]

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[ahrtr]

Thanks @jmhbnz !