No longer allow the x-elastic-product-origin header to be configurable

[kobelb]

Currently, users can override the value of the x-elastic-product-origin header that Kibana specifies on all HTTP requests to Elastic. Users should no longer be able to do so starting in 8.0, as this header is critical for the proper operation of Kibana's externally managed system indices per #81536

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[kobelb]

@joshdover can we get this added to the Core team's roadmap to be implemented by 8.0?

[pgayvallet]

So just to be sure, we are just talking about raising an error if "x-elastic-product-origin" is added to elasticsearch.requestHeadersWhitelist in the kibana config?

[kobelb]

@pgayvallet we should raise an error if "x-elastic-product-origin" is added to elasticsearch.requestHeadersWhitelist or elasticsearch.customHeaders.