update changelog

Author: ethandmd

CHANGELOG.md

@@ -13,7 +13,7 @@ Increment the:
 * MINOR version when you add functionality in a backwards compatible manner, and
 * PATCH version when you make backwards compatible bug fixes.
 
-## [1.21 2025-04-04]
+## [Unreleased]
 
 * [SDK] Base2 exponential histogram aggregation
   [#3175](https://github.com/open-telemetry/opentelemetry-cpp/pull/3346)
@@ -24,8 +24,6 @@ Increment the:
   ostream exporter, and otlp/grpc exporter. Updated histogram aggregation and
   benchmark tests.
 
-## [Unreleased]
-
 * [API] Remove `WITH_ABSEIL` and `HAVE_ABSEIL`
   [#3318](https://github.com/open-telemetry/opentelemetry-cpp/pull/3318)