etcdserver: add learner check to readyz

[gojoy]

fix #19001

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @gojoy. 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. Regular contributors should join the org to skip this step.

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