Fleet Server can only output to Elasticsearch. We've made one key change in #148896 wherein a policy containing the Fleet Server integration cannot have its output set to a Logstash output. However, we need to ensure that the same validation applies when an existing output is edited to have its type changed from Elasticsearch to Output.

If a Fleet Server policy exists that's pointing at an output, prevent its type attribute from being changed off of Elasticsearch. Users should be creating a separate output for their agents that output to Logstash, and this will prevent breaking their Fleet environment.