Docs: Add migration note about .yaml and .json removal
relates
relates
[[breaking_60_settings_changes]]
=== Settings changes
+==== Remove support for elasticsearch.json and elasticsearch.yaml configuration file
+
+The configuration file found in the Elasticsearch config directory could previously have
+a `.yml`, `.yaml` or `.json` extension. Only `elasticsearch.yml` is now supported.
+
==== Duplicate keys in configuration file
In previous versions of Elasticsearch, the configuration file was allowed to
