[release-3.5] Add write-only-skip-check option for --v2-deprecation to bypass the v2 content check

[sahilpatel09]

Originally solves #21250
This is a backport to v3.5 as mentioned in #21889: #21889 (comment)

I have not added tests just to keep it consistent with the previous PR.

[k8s-ci-robot]

Hi @sahilpatel09. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[sahilpatel09]

/cc @ahrtr

[ahrtr]

/ok-to-test

[ahrtr]

Manually verified.

$ ./bin/etcd --v2-deprecation=write-only-skip-check
......
{"level":"warn","ts":"2026-06-03T19:18:53.087187+0100","caller":"etcdserver/server.go:731","msg":"WARNING: --v2-deprecation=write-only-skip-check is enabled; v2 content detected, but validation check is being bypassed"}
......

[ahrtr]

LGTM, thx

Pls also add a changelog for 3.5.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, sahilpatel09

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