[DOCS] Fix Elastic Agent input examples

[renshuki]

ssl and ssl_verify_mode settings are deprecated for the Elastic Agent input (Logstash).
I adjusted the examples with current valid settings: ssl_enabled and ssl_client_authentication based of latest Logstash documentation: https://www.elastic.co/guide/en/logstash/current/plugins-inputs-elastic_agent.html#plugins-inputs-elastic_agent-ssl_client_authentication

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @renshuki? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kilfoyle]

LGTM! Thanks a lot for fixing this @renshuki!

[kilfoyle]

I'm not sure if you have merge permissions (the security permissions on all of our repos changed recently), so I'll merge this. I hope that's okay!

[renshuki]

I'm not sure if you have merge permissions (the security permissions on all of our repos changed recently), so I'll merge this. I hope that's okay!

Thanks @kilfoyle! 👍