chore: add changelog entry for swagger CpuConfig fix

Add an entry to mention that we fixed the Swagger API definition.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to
   unnecessary fields (`max_connections` and `max_pending_resets`) from the
   snapshot format, bumping the snapshot version to 5.0.0. Users need to
   regenerate snapshots.
+- [#4921](https://github.com/firecracker-microvm/firecracker/pull/4921): Fixed
+  swagger `CpuConfig` definition to include missing aarch64-specific fields.
 
 ### Deprecated