Roles Management - use privilege list from ES

[legrego]

Currently, Kibana sources its list of cluster and index privileges from a hard-coded list within Kibana itself.

Now that elastic/elasticsearch#29771 has been resolved via elastic/elasticsearch#42134, we should update Kibana to source these lists from Elasticsearch, via GET /_security/privilege/_builtin

[elasticmachine]

Pinging @elastic/kibana-security