So, my code looks precisely like the code in your gist.
Mind you, I am running on 8.2.2 - not even sure if 8.2.3 is available to me yet.
But, I get this exception
co.elastic.clients.elasticsearch._types.ElasticsearchException: [es/cat.indices] failed: [security_exception] unable to authenticate user [elastic] for REST request [/_cat/indices?format=json]
when I am executing a line from your code which I added:
String cat = client.cat().indices().toString();
Even there, authentication fails.
I am using the username "elastic" and the password given to me when I installed the system.