Update CHANGELOG.md

Author: xrmx

opamp/opentelemetry-opamp-client/CHANGELOG.md

@@ -7,9 +7,6 @@ Do *NOT* add changelog entries here!
 
 This changelog is managed by towncrier and is compiled at release time.
 
-The static "## Unreleased" section below pre-dates towncrier; its entries
-must be folded into the first towncrier-generated release manually.
-
 See https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/CONTRIBUTING.md#changelog for details.
 -->
 
@@ -18,8 +15,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- changelog start -->
 
-## Unreleased
-
 ## Version 0.2b0 (2026-04-01)
 
 - Breaking change: callback class `Callbacks` renamed to `OpAMPCallbacks`