Kibana version: 7.13+

Describe the bug:
Looking into https://www.elastic.co/guide/en/kibana/current/upgrade-assistant-api-default-field.html led me to dig in to the code a bit, which I believe is https://github.com/elastic/kibana/blob/v7.12.1/x-pack/plugins/upgrade_assistant/server/lib/query_default_field.ts . That code doesn't seem to exist from v7.13.0 onwards. Moreover, it doesn't look like the /api/upgrade_assistant/add_query_default_field/ route even exists anymore: https://github.com/elastic/kibana/tree/main/x-pack/plugins/upgrade_assistant/server/routes . When I try to hit that API it returns a 404. It seems that it was removed from the codebase but the documentation wasn't updated to reflect that. If that is the case, we should remove the documentation for versions 7.13 onwards.

Related: elastic/elasticsearch#69770