system-variables: add global scope to tidb_replica_read

[shichun-0415]

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• [x] master (the latest development version)
• [ ] v6.2 (TiDB 6.2 versions)
• [x] v6.1 (TiDB 6.1 versions)
• [x] v5.4 (TiDB 5.4 versions)
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What is the related PR or file link(s)?

• This PR is translated from: https://github.com/pingcap/docs/pull/6451
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• [ ] Delete files
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[ti-chi-bot]

[REVIEW NOTIFICATION]

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[shichun-0415]

/remove status LGT1 /status LGT2 /merge

[ti-chi-bot]

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

Commit hash: 581e468a1d04088be93a2d820dfca13777a84b1f

[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.