Combine node and cluster warnings in Upgrade Assistant

[joshdover]

Summary

We need to show all the node-level deprecations in the cluster tab. In 7.0, this Elasticsearch API may remove the node_settings completely and do this combination for us. At that time, we should remove the usage of the node_settings key.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-operations

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[tylersmalley]

This LGTM

Unrelated, the documentation page returned for the thread_pool size 404's cc: @gwbrown

This is running ES 6.6:

{
  "cluster_settings" : [ ],
  "node_settings" : [
    {
      "level" : "critical",
      "message" : "Index thread pool removed in favor of combined write thread pool",
      "url" : "https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking_70_cluster_changes.html#_index_thread_pool",
      "details" : "nodes with index thread pool settings: [xqd9QOH]"
    }
  ],
  "index_settings" : { }
}

[gwbrown]

@tylersmalley Thank you! I've opened elastic/elasticsearch#36854 to fix the doc links - there were others that were wrong as well.