v8.5.6 release notes: add info about cluster_id (#22766)

[ti-chi-bot]

This is an automated cherry-pick of #22766

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• This PR is translated from: v8.5.6 release notes: add info about cluster_id  docs-cn#21559
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[gemini-code-assist]

Code Review

This pull request updates the release notes for v8.5.6 by adding a section on system table changes, specifically documenting the addition of the cluster_id field to the mysql.tidb table. A review comment suggests improving the conciseness of the description by removing redundant phrasing regarding the read-only nature of the field and simplifying the sentence structure.

[qiancai]

/approve

[ti-chi-bot]

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