client/retry: only return the latest error in backoffer (#8227)

[ti-chi-bot]

This is an automated cherry-pick of #8227

What problem does this PR solve?

Issue Number: close #8499, ref #8142.

What is changed and how does it work?

Due to the return of historical errors causing the client's retry logic to fail,
and since we currently do not need to obtain all errors during retries, this PR
removes `multierr` from backoffer and add tests to ensure the correctness of the retry logic.

Check List

Tests

• Unit test
• Integration test

Release note

Fix the bug where HTTP client retry logic is ineffective.

[JmPotato]

There's no need to cherry-pick because it's a client-side change.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, JmPotato

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• OWNERS [HuSharp,JmPotato]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-06 10:46:54.626196386 +0000 UTC m=+348944.493295471: ☑️ agreed by JmPotato.
• 2024-08-06 10:50:20.686256766 +0000 UTC m=+349150.553355854: ☑️ agreed by HuSharp.