Remove old _knn_search tech preview API in v9

[benwtrent]

Removes the old _knn_search API that was never out of tech preview and deprecated throughout the v8 cycle.

To utilize the API, compatible-with=8 can be utilized.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @benwtrent, I've created a changelog YAML for you. Note that since this PR is labelled >breaking, you need to update the changelog YAML to fill out the extended information sections.

[john-wagster]

LGTM

[benwtrent]

@elastic/docs This removes a documentation page as the API is no longer generally accessible in v9.

Do we need to auto-forward the page? Can we do that in our docs?

[shainaraskas]

@benwtrent we do soft redirects by adding an entry to this page - that will unblock this PR. we then need to make a request to the web team to get a proper redirect put in place.

[mayya-sharipova]

Should we add a comment to remove this handler for V10?

[mayya-sharipova]

Thanks Ben, nice code cleanup

[leemthompo]

@benwtrent is this PR relevant to the serverless changelog? [FYI this question is based on 9.0 breaking changes]