Add replacement for cluster resource in "v1alpha1 to v1beta1 migration" docs

This is part of providing documentation for alternative to pipeline resource.
As we have now kubeconfig-creator task in the catalog as the replacement for cluster resource,
Added the same in the "v1alpha1 to v1beta1 migration" docs.
Ref: #1369

Signed-off-by: Divyansh42 <[email protected]>

Author: divyansh42

docs/migrating-v1alpha1-to-v1beta1.md

@@ -178,3 +178,7 @@ For examples of replacing an `image` resource, see the following Catalog `Tasks`
   illustrates how to write the digest of an image to a result.
 - The [Buildah Catalog `Task`](https://github.com/tektoncd/catalog/blob/v1beta1/buildah/)
   illustrates how to accept an image digest as a parameter.
+
+### Replacing a `cluster` resource
+
+You can replace a `cluster` resource with the [`kubeconfig-creator` Catalog `Task`](https://github.com/tektoncd/catalog/tree/v1beta1/kubeconfig-creator).