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
• rleungx

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[codecov]

Codecov Report

Patch coverage is 85.71% of modified lines.

❗ Current head fa2a3ad differs from pull request most recent head b8fd575. Consider uploading reports for the commit b8fd575 to get more accurate results

Files Changed	Coverage
server/schedule/region_scatterer.go	85.71%

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[bufferflies]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: fa2a3ad

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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