Update v1.5-v1.6 migration doc with deprecated code information

Signed-off-by: Furkat Gofurov <[email protected]>

docs/book/src/developer/providers/migrations/v1.5-to-v1.6.md

@@ -22,6 +22,11 @@ maintainers of providers and consumers of our Go API.
 ### Removals
 
 - API version `v1alpha4` is not served in v1.6 (users can enable it manually in case they are lagging behind with deprecation cycles). Important: `v1alpha4` will be completely removed in 1.7.
+- The function(s):
+    - `ClusterToObjectsMapper` is removed, please use `ClusterToTypedObjectsMapper` function instead.
+    - `Poll` and `PollImmediate` are removed, please use utils from "k8s.io/apimachinery/pkg/util/wait" instead.
+- The variable `DefaultIndexes` is removed, please use `[]Index{NodeProviderIDIndex}`
+- `ProviderID` type and all related methods/construct have been removed. Please see this [PR](https://github.com/kubernetes-sigs/cluster-api/pull/8577) for a reference.
 
 ### API Changes