*: 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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bufferflies
• nolouch

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[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
pkg/mock/mockcluster/mockcluster.go	94.74% <100.00%> (ø)
pkg/schedule/placement/rule_manager.go	85.89% <100.00%> (-0.41%)	⬇️
server/cluster/cluster.go	81.33% <100.00%> (ø)
server/config/persist_options.go	92.59% <100.00%> (+0.69%)	⬆️
server/server.go	73.90% <40.00%> (-0.41%)	⬇️

... and 27 files with indirect coverage changes

📢 Thoughts on this report? Let us know!.

[nolouch]

lgtm

[nolouch]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: d479546

[rleungx]

/merge

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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