v7.5.8 release notes: update the timeline table

[ti-chi-bot]

This is an automated cherry-pick of #21939

This is an automated cherry-pick of #21930

What is changed, added or deleted? (Required)

• Add releases/release-7.5.8.md for the TiDB 7.5.8 patch release notes.
• Add the TiDB 7.5.8 entry to TOC-tidb-releases.md, releases/_index.md, and releases/release-timeline.md.
• Fill in the TiDB 7.5.8 release date.
• This release-notes update is prepared on release-8.5 because the release-notes directory is maintained there and will be cherry-picked to master.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

• This PR is translated from: add v7.5.8 release notes docs#23757
• Other reference link(s): generated from doc_4afdcdb19e49_release-7.5.8-updated-by-ai.md

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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

Summary by CodeRabbit

• 文档
  • 调整 TiDB 版本发布时间线中 7.5.8 记录的位置，使其按发布日期倒序排列。
  • 保留发布日期（2026 年 9 月 17 日）及对应版本详情链接。

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ℹ️ Recent review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between 7d591ec and 62ff4cc.

📒 Files selected for processing (1)

• releases/release-timeline.md

💤 Files with no reviewable changes (1)

• releases/release-timeline.md

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📝 Walkthrough

Walkthrough

发布时间线将现有 7.5.8 记录移至首行，使记录按发布日期倒序排列。记录内容保持不变。

Changes

版本发布时间线

Layer / File(s)	Summary
调整发布时间线排序 releases/release-timeline.md	将现有 7.5.8 记录从 8.5.4 之后移至 8.5.8 之前。其余记录保持不变。

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Other

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	标题清晰说明了主要变更，即更新 v7.5.8 发布说明中的时间线表格。标题与变更内容相关且足够具体。
Description check	✅ Passed	描述包含变更内容、适用版本、相关 PR、AI 参与情况和变更类型检查项。必需章节基本完整。描述中的版本复选项与模板版本列表存在差异，但已正确勾选 v7.5，且不影响本次变更的说明。

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

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[qiancai]

/approve

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