*: 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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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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[codecov]

Codecov Report

Attention: Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.55%. Comparing base (f4d1cb7) to head (a179f10).
Report is 9 commits behind head on release-6.1.

Files with missing lines	Patch %	Lines
server/config/persist_options.go	0.00%	3 Missing ⚠️
server/server.go	40.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-6.1    #7125      +/-   ##
===============================================
+ Coverage        75.46%   75.55%   +0.08%     
===============================================
  Files              307      307              
  Lines            30589    30594       +5     
===============================================
+ Hits             23085    23116      +31     
+ Misses            5506     5483      -23     
+ Partials          1998     1995       -3     

Flag	Coverage Δ
unittests	75.55% <45.45%> (+0.08%)	⬆️

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[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 07:25:10.777409551 +0000 UTC m=+341180.517833489: ☑️ agreed by lhy1024.
• 2024-09-10 08:24:18.930328013 +0000 UTC m=+344728.670751952: ☑️ agreed by HuSharp.