Update changelog with changes from v2.21.0

Author: newrelic-coreint-bot

CHANGELOG.md

@@ -9,7 +9,9 @@ Unreleased section should follow [Release Toolkit](https://github.com/newrelic/r
 
 ## Unreleased
 
-### enhancements
+## v2.21.0 - 2025-09-09
+
+### 🚀 Enhancements
 - Reduced QueryMonitoringResponseTimeThreshold from 500ms to 1ms to improve visibility of Individual query performance data immediately
 
 ## v2.20.3 - 2025-08-26