[DOCS] Advise users to migrate from Filebeat to Elastic Agent (#36157)

## What does this PR do?

Adds a note to
[Modules](https://www.elastic.co/guide/en/beats/filebeat/current/filebeat-modules.html)
to advise users to migrate to Elastic Agent.

Preview:
[Modules](https://beats_36157.docs-preview.app.elstc.co/guide/en/beats/filebeat/master/filebeat-modules.html)

## Why is it important?

While Filebeat modules are still supported, we do not typically apply
bug fixes or enhancements to these modules, or ship any new modules. Our
focus is now entirely on Elastic Agent integrations.

## Checklist

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~~- [ ] I have added an entry in `CHANGELOG.next.asciidoc` or
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## Related issues

* Resolves elastic/security-docs#3576

---------

Co-authored-by: Janeen Mikell Roberts <[email protected]>
Co-authored-by: David Kilfoyle <[email protected]>
(cherry picked from commit e1267be)

filebeat/docs/modules.asciidoc

@@ -9,4 +9,6 @@ modules.
 
 Filebeat modules require Elasticsearch 5.2 or later.
 
+NOTE: While {filebeat} modules are still supported, we recommend {agent} integrations over {filebeat} modules. Integrations provide a streamlined way to connect data from a variety of vendors to the {stack}. Refer to the https://www.elastic.co/integrations/data-integrations[full list of integrations]. For more information, please refer to the {fleet-guide}/beats-agent-comparison.html[{beats} vs {agent} comparison documentation].
+
 include::modules_list.asciidoc[]