server: fix the leader cannot election after pd leader lost while etcd leader intact (#6447)

[ti-chi-bot]

This is an automated cherry-pick of #6447

What problem does this PR solve?

Issue Number: Close #6403

What is changed and how does it work?

server: fix the leader cannot election after pd leader lost while etcd leader intact

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• nolouch

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: e7afc55

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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