Update changelog with changes from v3.15.0

Author: newrelic-coreint-bot

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
-### security
+## v3.15.0 - 2026-05-20
+
+### 🛡️ Security notices
 - Upgraded `go.opentelemetry.io/otel` from v1.40.0 to v1.41.0 to fix [GHSA-mh2q-q3fh-2475](https://github.com/advisories/GHSA-mh2q-q3fh-2475) (High severity)
 
 ## v3.14.0 - 2026-05-18