Availability of cluster.stats in Serverless

[picandocodigo]

The definition for cluster.stats is out of sync with Serverless.

The source code labels it as stable/public.

* @availability serverless stability=stable visibility=public

However, when running this API against Serverless we get a 404 error:

>client.cluster.stats
[404] {"error":"Request for uri [/_cluster/stats] with method [GET] exists but is not available when running in serverless mode"} 
(Elastic::Transport::Transport::Errors::NotFound)