address review comments

Author: sbueringer

docs/book/src/developer/providers/migrations/v1.4-to-v1.5.md

@@ -24,6 +24,7 @@ maintainers of providers and consumers of our Go API.
 ### Removals
 
 - API version `v1alpha3` is not served in v1.5 (users can enable it manually in case they are lagging behind with deprecation cycles). Important: `v1alpha3` will be completely removed in 1.6.
+- The lazy restmapper feature gate was removed in controller-runtime and lazy restmapper is now the default restmapper. Accordingly the `EXP_LAZY_RESTMAPPER` feature gate was removed in Cluster API. 
 
 ### API Changes