release-8.5: add v8.5.7 release notes (#21764) (#21783)

Author: ti-chi-bot

TOC-tidb-releases.md

@@ -12,6 +12,7 @@
 ## 版本发布说明
 
 - v8.5
+  - [8.5.7](/releases/release-8.5.7.md)
   - [8.5.6](/releases/release-8.5.6.md)
   - [8.5.5](/releases/release-8.5.5.md)
   - [8.5.4](/releases/release-8.5.4.md)

releases/_index.md

@@ -14,6 +14,7 @@ aliases: ['/zh/tidb/stable/release-notes/','/zh/tidb/v8.5/release-notes/']
 
 ## 8.5
 
+- [8.5.7](/releases/release-8.5.7.md): 2026-07-09
 - [8.5.6](/releases/release-8.5.6.md): 2026-04-14
 - [8.5.5](/releases/release-8.5.5.md): 2026-01-15
 - [8.5.4](/releases/release-8.5.4.md): 2025-11-27