[BUG] Obsolete theme:version config value of v8 (beta) rendering issues

[kavilla]

Describe the bug

The theme was marked hidden in Advanced Settings within the UI but is still can be utilized and configurable. Users who have originally set the theme version from v7 (or unset) to v8 (beta) experienced rendering issues within OpenSearch Dashboards. For example, bread crumbs would render like this on v8 (beta):

Reporting external plugin would render as:

Dashboards Version
2.x

Temp workaround
In 2.x version you can verify if your OpenSearch Dashboards utilized the v8 theme by running the following command in the Dev Tools, replacing $VERSION with your current version of the application:

GET .kibana/_doc/config:$VERSION

Even though the UI setting was removed in 2.x you can still update the setting by navigating to Dev Tools and pasting the following script:

POST .kibana*/_update_by_query
{
  "query": {
    "term": {
      "type": {
        "value": "config"
      }
    }
  },
  "script" : "ctx._source.config.remove('theme:version')"
}

Then refresh the application.

---

Curl commands of the above statement:

curl -XGET "http://localhost:9200/.kibana/_doc/config:$VERSION"

curl -XPOST "http://localhost:9200/.kibana*/_update_by_query" -H 'Content-Type: application/json' -d'
{
  "query": {
    "term": {
      "type": {
        "value": "config"
      }
    }
  },
  "script" : "ctx._source.config.remove('\''theme:version'\'')"
}'

---

Permanent solution
We should create a migration to remove this value automatically from the system index as this configuration should no longer be used.

s/o @AMoo-Miki, @manasvinibs, @joshuarrrr, @BSFishy for the help here.

[tmarkley]

Related: do we still want to address #494?