backup: change the location for s3-multi-part-size (#11568)

[ti-chi-bot]

This is an automated cherry-pick of #11568

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What is changed, added or deleted? (Required)

change the location of s3-multi-part-size from causal-ts to backup

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For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v6.4 (TiDB 6.4 versions)
• v6.3 (TiDB 6.3 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• shichun-0415

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[shichun-0415]

/merge

[ti-chi-bot]

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

Commit hash: 2962ff4

[shichun-0415]

/verify