Update warning for Elastic Defend with Remote ES cluster

[kilfoyle]

This updates the warning on the Remote Elasticsearch output page about using Elastic Defend.

Closes: elastic/security-docs#6711
Note: Once merged we'll need an equivalent PR for the 9.0 docs.

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[ferullo]

@caitlinbetz @dasansol92 can you review this?

EDIT: FWIW, I think we should say less (just say Elastic Defend won't work right in Kibana with remote output). It's not that I think we should avoid saying what doesn't work but worry there might be other things we don't know to mention and also this seems like a lot to mention in a "tip".

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Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[kilfoyle]

@ferullo I've updated the note to remove details and just say that Elastic Defend isn't supported. The PR description shows the latest.

[ferullo]

👍 It looks good to me but I think either @caitlinbetz or @roxana-gheorghe should 👍 too before merge. Thanks!

[karenzone]

LGTM