br: use merge-schedule-limit to pause merge (#47576)

[ti-chi-bot]

This is an automated cherry-pick of #47576

What problem does this PR solve?

Issue Number: ref tikv/pd#7148

Problem Summary:
max-merge-region-keys is not a stable arguments.
merge-schedule-limit is more stable to control the merge operator.

What is changed and how it works?

use merge-schedule-limit instead of other arguments to control the merge operator during restore.

Check List

Tests

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

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	75c415c	link	true	/test unit-test

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