system-variables: add global scope to tidb_replica_read

[shichun-0415]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

This PR only aligns one modification from the source PR (pingcap/docs#6451) because other contents are already there in the docs-cn repo.

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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)
• v6.2 (TiDB 6.2 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)

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

• This PR is translated from: system-variables: update from generated source docs#6451
• Other reference link(s):

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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• Oreoxmt

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[Oreoxmt]

LGTM

[shichun-0415]

/remove status LGT1
/status LGT2
/merge

[ti-chi-bot]

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

Commit hash: 581e468

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10928.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10929.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10930.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10933.