Update etcd status and leadership

[jberkus]

Checklist for maintainer updates

• You've provided a link to documentation where the project has approved the maintainer changes.
• The maintainer(s) also created or updated their LFX Individual Dashboard profile.
• You've sent an email with the list of email address(es) to cncf-maintainer-changes@cncf.io for invitations to Service Desk and mailing lists. You can just mark this complete if you are only removing people.
• Optional: You've also sent a PR with affiliation updates to cncf/gitdm.

Changes to reflect current etcd status and membership:

1. etcd is part of Kubernetes now, and is SIG-etcd. As such, the separate Etcd project listing (which is mostly the same people) is being deleted. As such, all SIG-etcd leaders are being added under Kubernetes Maintainers.
2. Current SIG-etcd leadership can be found in SIGS file.

@ahrtr @siyuanfoundation @ivanvc @serathius

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