TEST ONLY - DO NOT MERGE: Validate ECS attr for LSR

[karenzone]

DO NOT MERGE

Test set to accompany elastic/logstash#13427 elastic/logstash#13462
This PR adds the {ecs-default} attribute to output-elasticsearch.asciidoc to validate that the attribute works as expected.

TEST NOTES

• Broken cross-ref. This PR will fail docs CI because the {logstash-ref} attribute used in the default attribute is looking for the master branch, which has been renamed to main. ToDo: Find out why and fix it.
  (I'm putting up the PR anyway to have a place to put notes and findings.)

• The attribute text also needs to be updated to display link text instead of url.

• Here's how the attribute is rendering locally:

  

  I formatted it as a bullet to match the default format for other config options. I might reconsider that.

[karenzone]

I told you docs-ci was going to fail and why. 😉

18:28:33 INFO:build_docs:Bad cross-document links:
18:28:33 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/logstash/master/plugins-outputs-elasticsearch.html contains broken links to:
18:28:33 INFO:build_docs: - en/logstash/master/ls-ecs

[karenzone]

This Test PR will fail docs-ci until #13462 is merged.

[karenzone]

@elasticmachine run elasticsearch-ci/docs