Doc: Update security docs to replace obsolete cacert setting

[karenzone]

Related:

• Issue: Remove deprecated SSL settings from Elasticsearch output logstash-plugins/logstash-output-elasticsearch#1190
• PR: Mark previously deprecated SSL settings as obsolete logstash-plugins/logstash-output-elasticsearch#1197

The "Secure your connection to Elasticsearch > Configure the elasticsearch output" topic contains inaccurate info (and link) in light of obsolete SSL setttings.

This PR updates the content to point to replacement setting and fixes the example.

PREVIEW: https://logstash_bk_16798.docs-preview.app.elstc.co/guide/en/logstash/master/ls-security.html#es-sec-plugin

[karenzone]

This PR must be approved/merged before we can merge logstash-plugins/logstash-output-elasticsearch#1197.

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_16798.docs-preview.app.elstc.co/diff

[karenzone]

@donoghuc, this doc change is a prereq for logstash-plugins/logstash-output-elasticsearch#1197. Will you take a look and make sure that I didn't miss anything, please?

[donoghuc]

Good catch! as part of #16652 i'm searching the LS repo for any mentions of deprecated ssl related values. I'll let you know if i find more in the docs.

I'm not sure style wise if we prefer to have it explicitly as a list

ssl_certificate_authorities => ['/etc/logstash/config/certs/ca.crt']

But it certainly appears to work fine as just a single value. Here is the spec https://github.com/logstash-plugins/logstash-output-elasticsearch/blob/6a32bbe2ac02ee9491bc734b1cf4aa816c161237/lib/logstash/plugin_mixins/elasticsearch/api_configs.rb#L71

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_16798.docs-preview.app.elstc.co/diff

[karenzone]

18765

[karenzone]

@logstashmachine backport 8.x

[karenzone]

@logstashmachine backport 8.17

[karenzone]

@logstashmachine backport 8.16