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

[LykxSassinator]

This reverts commit aa85b6c0047b0f2af37c1bd271ee194af1c0dd56.

What problem does this PR solve?

Issue Number: Ref https://github.com/pingcap/tidb/issues/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

[ti-chi-bot[bot]]

@LykxSassinator: The /test command needs one or more targets. The following commands are available to trigger required jobs:

• /test build

The following commands are available to trigger optional jobs:

• /test pull-integration-copr-test

Use /test all to run the following jobs that were automatically triggered:

• tikv/pd/ghpr_build

In response to [this](https://github.com/tikv/pd/pull/8541#issuecomment-2292757904): >/test Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[LykxSassinator]

/test all

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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](https://app.codecov.io/gh/tikv/pd/pull/8541/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/tikv/pd/pull/8541/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv) | `77.43% <ø> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#carryforward-flags-in-the-pull-request-comment) to find out more.

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

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

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tikv/pd/blob/release-8.3/OWNERS)~~ [HuSharp,lhy1024,nolouch] - ~~[conf/OWNERS](https://github.com/tikv/pd/blob/release-8.3/conf/OWNERS)~~ [niubell] - ~~[pkg/schedule/config/OWNERS](https://github.com/tikv/pd/blob/release-8.3/pkg/schedule/config/OWNERS)~~ [niubell] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment