Do not include root level _type field if it is not a metadata field

[salvatore-campagna]

_type is a (deprecated) metadata field. When a non-metadata field is created whose name
is type, it must be included in the _source field but not as a root-level field, as done
for type when it is a metadata field. For Elasticsearch using version 7 of the REST api the
type field is, indeed, a metadata field which is included at the root level of the response.

Resolves #110438

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[elasticsearchmachine]

Hi @salvatore-campagna, I've created a changelog YAML for you.