[ElasticAgent] Update Logstash output doc to use most current ssl settings

[karenzone]

Configuration example uses outdated security settings, specifically cacert and force-peer

Topic: https://www.elastic.co/docs/reference/fleet/logstash-output
Source: https://github.com/elastic/docs-content/blame/main/reference/fleet/logstash-output.md

Resources and notes

• https://www.elastic.co/docs/reference/logstash/plugins/plugins-inputs-elastic_agent
  • ssl_verify_mode replaced by ssl_client_authentication
• https://www.elastic.co/docs/reference/logstash/plugins/plugins-outputs-elasticsearch
  • cacert replaced by ssl_certificate_authorities

Related

Scrub docsets for remaining instances of cacert (docs-content, logstash, beats repos)