system-variable: the value of ‘tidb_service_scope’ becomes case-insensitive in v9.0.0 and v8.5.6 (#21422)

[ti-chi-bot]

This is an automated cherry-pick of #21422

Update variable description of ‘tidb_service_scope’

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• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

Update system-variables ‘tidb_service_scope’

Which TiDB version(s) do your changes apply to? (Required)

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By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

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

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):https://github.com/pingcap/docs-cn/pull/21478/files#diff-ec2cbb115cc58fe3afe2862eb5148732d323cbf7ae35dea97bd381a1f8d0e8a0

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]

@Frank945946 This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[hfxsd]

/approve

[hfxsd]

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[hfxsd]

/approve

[hfxsd]

/approve

[ti-chi-bot]

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