chore: move changelog entry

Author: 43081j

CHANGELOG.md

@@ -10,8 +10,6 @@ For notes on migrating to 2.x / 0.200.x see [the upgrade guide](doc/upgrade-to-2
 
 ## Unreleased
 
-* perf(opentelemetry-core): optimize attribute serialization [#5866](https://github.com/open-telemetry/opentelemetry-js/pull/5866) @43081j
-
 ### :boom: Breaking Changes
 
 ### :rocket: Features
@@ -33,6 +31,7 @@ For notes on migrating to 2.x / 0.200.x see [the upgrade guide](doc/upgrade-to-2
 
 * test(shim-opentracing): add comparison thresholds in flaky assertions [#5974](https://github.com/open-telemetry/opentelemetry-js/pull/5974) @cjihrig
 * test(exporter-jaeger): clean up OTEL_EXPORTER_JAEGER_AGENT_PORT between tests [#6003](https://github.com/open-telemetry/opentelemetry-js/pull/6003) @cjihrig
+* perf(opentelemetry-core): optimize attribute serialization [#5866](https://github.com/open-telemetry/opentelemetry-js/pull/5866) @43081j
 
 ## 2.1.0