Revert "*: enlarge the default value of max-merge-region-size. (#8445)"

[LykxSassinator]

This reverts commit aa85b6c.

What problem does this PR solve?

Issue Number: Ref pingcap/tidb#55374

What is changed and how does it work?

Revert the changes on the configuration of region-size. These changes will be delayed until v8.4.

Check List

Tests

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

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[LykxSassinator]

/test

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[LykxSassinator]

/test all

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.43%. Comparing base (248206b) to head (3ec6a99).
Report is 1 commits behind head on release-8.3.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.3    #8541      +/-   ##
===============================================
+ Coverage        77.41%   77.43%   +0.01%     
===============================================
  Files              473      473              
  Lines            61933    61933              
===============================================
+ Hits             47946    47955       +9     
- Misses           10406    10410       +4     
+ Partials          3581     3568      -13     

Flag	Coverage Δ
unittests	77.43% <ø> (+0.01%)	⬆️

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-16 04:21:36.576814737 +0000 UTC m=+500981.280284395: ☑️ agreed by nolouch.
• 2024-08-16 07:00:20.696965368 +0000 UTC m=+510505.400435011: ☑️ agreed by lhy1024.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [HuSharp,lhy1024,nolouch]
• conf/OWNERS [niubell]
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