ctl: Set ETCDCTL_API=v3 to get the v3 API

[dveeden]

What problem does this PR solve?

Ensure tiup ctl etcd works with TiDB by default.

Closes #2080

What is changed and how it works?

Check List

Tests

• Manual test (add detailed scripts or steps below)

Release notes:

NONE

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[nexustar]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

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Commit hash: 23ddf4e

[ti-chi-bot]

@dveeden: Your PR was out of date, I have automatically updated it for you.

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