[UA] The `update_index` API in upgrade

[jloleysens]

There is an edge case where deprecated 7.x index settings can still exist in 8.x: when users do not use the UA and go straight to 8.x without first cleaning up their settings. The UA's update_index API does not clean up these "old settings" despite the UI saying that it will:

When we removed the "remove deprecated settings" functionality from Kibana we were attempting to address some tech debt that Kibana should not need to maintain the list of deprecated index settings and know how to remove them.

With the introduction of the "mark read-only" flow for regular indices, we now skip this step in our update_index API, even though the UI still suggests that these deprecated settings will be removed.

As I see it there 2 ways to address this:

1. We reinstate some functionality in Kibana that did not carry over to the read-only flow
2. We find a way to handle this at the ES level, perhaps just raising these settings as their own deprecations? In which case the Kibana UI could be deleted or remain, but it is just misleading at the moment

IMO (2) is a bit nicer, as (1) means Kibana needs to maintain its own list of deprecated settings in ES which is something we are trying to move away from.

CC @masseyke @parkertimmins (perhaps either of you could help guide us here).

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)

[jloleysens]

Example of index settings that could lead to this:

    "settings": {
      "index": {
        "routing": {
          "allocation": {
            "include": {
              "_tier_preference": "data_content"
            }
          }
        },
        "number_of_shards": "2",
        "force_memory_term_dictionary": "true",
        "max_adjacency_matrix_filters": "100",
        "provided_name": "deprecated_settings",
        "mapper": {
          "dynamic": "false"
        },
        "creation_date": "123",
        "number_of_replicas": "1",
        "uuid": "abc",
        "version": {
          "created": "7172799"
        }
      }
    }
  }