Update changelog [skip ci]

Author: leburgel

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add unit normalization of the half and full infinite environments before the projector
   computation, fixing an issue with the gradient accuracy.
+- Fix sporadic test timeouts when running variational optimization after simple
+  update for the Heisenberg model by switching to a `GMRES`-based gradient solver.
 
 ### Changed