Adds to agentless FAQ about upgrading agentless integrations (#3108)

Fixes #3029 by adding a new FAQ item about upgrading agentless
integrations to new versions. There's a fix for 9.2, but prior to 9.2
the UI wouldn't allow you to upgrade an existing integration, other than
by deleting and reinstalling it. I decided not to mention serverless in
the FAQ item because the upgrade issue has been fixed in serverless.

Author: benironside

solutions/security/get-started/_snippets/agentless-integrations-faq.md

@@ -27,6 +27,9 @@ To troubleshoot this issue:
 If the **Make this {{fleet}} server the default one** setting was already enabled but problems persist, it’s possible someone changed the default {{fleet}} server’s **URL** value. In this case, contact Elastic Support to find out what the original **URL** value was, update the settings to match this value, then delete your integration and create it again.
 ::::
 
+## Why can't I upgrade my agentless integration to a newer version?
+
+On versions of {{stack}} before v9.2, agentless integrations can't be upgraded to newer versions of the integration. To get a newer version in your {{stack}} environment, upgrade to {{stack}} v9.2+ or delete and re-install the desired integration.
 
 
 ## How do I troubleshoot an `Unhealthy` agent? [_how_do_i_troubleshoot_an_unhealthy_agent]