*: fix sync isolation level to default placement rule (#7122)

[ti-chi-bot]

This is an automated cherry-pick of #7122

What problem does this PR solve?

Issue Number: Close #7121.

What is changed and how does it work?

This PR will sync the isolation level to the default rule when setting config with HTTP API or pd-ctl.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

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[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[rleungx]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, lhy1024

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Needs approval from an approver in each of these files:

• OWNERS [HuSharp,lhy1024]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-10 08:25:17.917075428 +0000 UTC m=+344787.657499366: ☑️ agreed by HuSharp.
• 2024-09-10 08:37:49.182758889 +0000 UTC m=+345538.923182823: ☑️ agreed by lhy1024.

[purelind]

/hold

[purelind]

/unhold