Replace nested two-level lists with bold section headers in changelogs

[Copilot]

Changelog files used a two-level list structure where top-level items served as section headings and second-level items held the actual entries. This converts all such files to use bold inline headers (**Section**) followed by flat lists.

Before:

- 启动器
    - GP-5851: 将 kala-compress 更新至 1.27.1-3 (by Glavo)
    - GP-5854: 将 twelvemonkeys 更新至 3.13.1 (by Glavo)
- 错误修复
    - GP-5910: 修复无法为 Minecraft 26.1.1 从官方源安装 NeoForge 的问题 (by Glavo)

After:

**启动器**

- GP-5851: 将 kala-compress 更新至 1.27.1-3 (by Glavo)
- GP-5854: 将 twelvemonkeys 更新至 3.13.1 (by Glavo)
**错误修复**

- GP-5910: 修复无法为 Minecraft 26.1.1 从官方源安装 NeoForge 的问题 (by Glavo)

Affected files — 14 files across _changelogs/stable/:

• 3.6/3.6.16.md, 3.6.17.md, 3.6.18.md, 3.6.20.md
• 3.7/3.7.5.md, 3.7.6.md
• 3.8/3.8.1.md, 3.8.2.md
• 3.9/3.9.1.md, 3.9.2.md
• 3.10/3.10.1.md, 3.11/3.11.1.md, 3.12/3.12.4.md, 3.13.1/README.md

Existing blank lines between sections and multi-line list item continuations are preserved.