book: add missing apiversion deprecation to migration docs

Author: oscr

docs/book/src/developer/providers/v1.3-to-v1.4.md

@@ -19,7 +19,11 @@ maintainers of providers and consumers of our Go API.
 
 ### Deprecation
 
--
+- The api versions `v1alpha3` and `v1alpha4` are deprecated and will be removed. 
+  - `v1alpha3` will be removed in v1.5 
+  - `v1alpha4` will be removed in v1.6
+
+  For more information, please see: https://release-1-4.cluster-api.sigs.k8s.io/contributing?highlight=v1alpha#removal-of-v1alpha3--v1alpha4-apiversions
 
 ### Removals