Today, some Elasticsearch REST endpoints (e.g., update index settings API) attempt to determine the content type (e.g., JSON vs. YAML) of the request body with a rudimentary detection mechanism. This mechanism breaks down on YAML bodies that contain braces because the content is inappropriately detected as JSON. Instead, this mechanism should not be used, Elasticsearch should default to JSON for all request bodies, and possibly obey the Content-Type header for allowing YAML.

Relates #19366