Update changelog with changes from v1.21.0

Author: newrelic-coreint-bot

CHANGELOG.md

@@ -9,9 +9,12 @@ Unreleased section should follow [Release Toolkit](https://github.com/newrelic/r
 
 ## Unreleased
 
+## v1.21.0 - 2026-04-27
+
+### 🛡️ Security notices
+- update go to v1.25.8
+- update filippo.io/edwards25519 to v1.1.1
 
-### security
-- update go to v1.25.9
 ## v1.20.0 - 2026-03-26
 
 ### 🛡️ Security notices