Scatter: make peer scatter logic same with the leader (#6965)

[ti-chi-bot]

This is an automated cherry-pick of #6965

What problem does this PR solve?

Issue Number: Close #6962

What is changed and how does it work?

In past, PD conside peer distribution in the different group influenece by using `TotalCountByStore` , but not include the leader distribution. The max used situation is partition table. 
After this pr, TIDB call scatter api will use same group not different. 

ref:
https://github.com/tikv/pd/pull/3422
https://github.com/pingcap/tidb/pull/46156

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

Side effects

Related changes

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bufferflies
• disksing

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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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[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5cf2c43) 75.43% compared to head (f616401) 75.47%.

Files	Patch %	Lines
server/schedule/region_scatterer.go	85.71%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-6.1    #7026      +/-   ##
===============================================
+ Coverage        75.43%   75.47%   +0.03%     
===============================================
  Files              308      308              
  Lines            30603    30591      -12     
===============================================
+ Hits             23085    23088       +3     
+ Misses            5514     5501      -13     
+ Partials          2004     2002       -2     

Flag	Coverage Δ
unittests	75.47% <85.71%> (+0.03%)	⬆️

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[bufferflies]

/merge

[ti-chi-bot]

@bufferflies: 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: dd2a408

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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