[3.5] docs: remove warning that etcd 3.5.0 was not yet released

[ghouscht]

See description in #18410

[k8s-ci-robot]

Hi @ghouscht. Thanks for your PR.

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[ivanvc]

/ok-to-test

[ivanvc]

LGTM. Thanks, Thomas.

[ahrtr]

Thanks for the cleanup!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, ghouscht, ivanvc, serathius

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[ivanvc]

/retitle [3.5] docs: remove warning that etcd 3.5.0 was not yet released