add v6.5.12 release notes (pingcap#20347) (pingcap#20399)

Author: ti-chi-bot

TOC.md

@@ -1114,6 +1114,7 @@
   - v6.6
     - [6.6.0-DMR](/releases/release-6.6.0.md)
   - v6.5
+    - [6.5.12](/releases/release-6.5.12.md)
     - [6.5.11](/releases/release-6.5.11.md)
     - [6.5.10](/releases/release-6.5.10.md)
     - [6.5.9](/releases/release-6.5.9.md)