Add breaking change entry

Author: dnhatn

docs/release-notes/breaking-changes.md

@@ -14,6 +14,11 @@ To learn how to upgrade, check out <uprade docs>.
 
 % ## Next version [elasticsearch-nextversion-breaking-changes]
 
+## 9.1.0 [elasticsearch-910-breaking-changes]
+
+ES|QL
+:   * Allow partial results by default in ES|QL [#125060](https://github.com/elastic/elasticsearch/pull/125060)
+
 ## 9.0.0 [elasticsearch-900-breaking-changes]
 
 Allocation