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

[Frank945946]

Update variable description of ‘tidb_service_scope’

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What is changed, added or deleted? (Required)

Update system-variables ‘tidb_service_scope’

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• 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)

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• Delete files
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@Frank945946: 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
pull-verify	c91463e	link	true	/test pull-verify

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