backup: add time zone in examples (#17705)

[ti-chi-bot]

This is an automated cherry-pick of #17705

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What is changed, added or deleted? (Required)

Add more details about backupts with time zone.

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• [x] v8.2 (TiDB 8.2 versions)
• [x] v8.1 (TiDB 8.1 versions)
• [x] v8.0 (TiDB 8.0 versions)
• [x] v7.6 (TiDB 7.6 versions)
• [x] v7.5 (TiDB 7.5 versions)
• [x] v7.1 (TiDB 7.1 versions)
• [x] v6.5 (TiDB 6.5 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)

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• [ ] Delete files
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• [ ] Might cause conflicts after applied to another branch

[Oreoxmt]

/approve

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