[OAS] Include x-availability-since for on-prem Kibana

[jloleysens]

Original context, quoting @lcawl :

"
The Elasticsearch specifications now have a cool x-available-since qualifer per elastic/elasticsearch-specification#2655. Not sure how/if that could be added to the Kibana automation, but seems like a really cool bit of info for us to include in the docs.
...
...optionally added to the specification (which for Elasticsearch is maintained separately from the code) and is only applicable to on-prem per its description in https://github.com/elastic/elasticsearch-specification/blob/main/docs/add-new-api.md. I presume it's not necessary in serverless since we have version info in the headers. However in the on-prem scenario, I'm investigating whether this info in an x-availability-since property helps us get to a goal of fewer API doc versions.
"

[elasticmachine]

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

[jloleysens]

Not 100% clear (yet) this is something that will turn into a work item for the platform team at this stage, but I can see how we may want input from API owners in determining "available since".

@lcawl Is it possible this information could be captured in an overlay and the stack version scraped from git blame? Might be a bit tedious, but seems possible to do this for sets of APIs.