Add back private knn_search API for rest-api-spec generation (#5737) (#5742)

* Revert "Remove knn_search API (#4276)"

This reverts commit 2a24e33 since _knn_search
is still needed in rest-api-spec to run YAML tests against it, as the API is
still available with v8 compatibility mode.

However, we make the API private so that it does not get back into clients, as
it would fail with v9 compatibility.

* Add UpdateForV10 annotation

* Fix lint

(cherry picked from commit 709ff24)

Co-authored-by: Quentin Pradet <quentin.pradet@elastic.co>

Author: github-actions[bot]

compiler/src/model/utils.ts

@@ -1074,7 +1074,9 @@ export function parseJsDocTags (jsDoc: JSDoc[]): Record<string, string> {
         value: tag.getComment() ?? ''
       }
     })
-  const mapped = tags.reduce((acc, curr) => ({ ...acc, [curr.name]: curr.value }), {})
+  // Ignore UpdateForV10 which is only useful at the eslint level
+  const filteredTags = tags.filter(tag => tag.name !== 'UpdateForV10')
+  const mapped = filteredTags.reduce((acc, curr) => ({ ...acc, [curr.name]: curr.value }), {})
   return mapped
 }