In #53734, we decided to rename the "system API" concept to "system request".

The header that is used was also renamed from kbn-system-api to kbn-system-request to better match the code. The backend will continue to support kbn-system-api for legacy plugin support and to support the use case where the client is running an older version than the server, where requests may still contain the kbn-system-api header.

In 8.1+ we can remove support for the old kbn-system-api header.