Update the scope of system-variable ”tidb_redact_log“

[Frank945946]

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What is changed, added or deleted? (Required)

Updated the scope of tidb_redact_log from "SESSION | GLOBAL" to "GLOBAL".

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[lilin90]

/lgtm

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[LGTM Timeline notifier]

Timeline:

• 2025-08-06 07:04:22.249334619 +0000 UTC m=+424572.392095794: ☑️ agreed by lilin90.

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