[chore]: update the CHANGELOG

Author: dottorblaster

CHANGELOG.md

@@ -22,6 +22,7 @@ the release.
   ([#2644](https://github.com/open-telemetry/opentelemetry-demo/pull/2644))
 * [frontend] fix item calculation and shipping
   ([#2684](https://github.com/open-telemetry/opentelemetry-demo/pull/2684))
+* [flagd-ui] add back legacy REST APIs to empower programmatic usage ([#2720](https://github.com/open-telemetry/opentelemetry-demo/pull/2720))
 
 ## 2.1.3