[DOCS] Document a known issue for agentless integrations (#242307)

## Summary

This PR documents a known issue that affects the connectivity between
agentless integrations and Fleet Server in Elastic Cloud Hosted
deployments.

Related to elastic/docs-content#3853
Contributes to closing
elastic/ingest-docs#1869

---------

Co-authored-by: florent-leborgne <florent.leborgne@elastic.co>

Author: vishaangelova

docs/release-notes/known-issues.md

@@ -8,6 +8,21 @@ For Elastic {{observability}} known issues, refer to [Elastic Observability know
 
 For Elastic Security known issues, refer to [Elastic Security known issues](docs-content://release-notes/elastic-security/known-issues.md).
 
+::::{dropdown} The connection between agentless integrations and {{fleet-server}} is broken
+:applies_to: ess: ga
+
+Applies to: {{stack}} <9.1.6
+
+**Details**
+
+When agentless integrations are deployed in {{fleet}} in an {{ech}} deployment, the connection between the agentless integrations and {{fleet-server}} can be broken if the default {{fleet-server}} host URL is modified or if a different host URL is set as the default.
+
+**Resolved**
+
+This issue is resolved in {{stack}} 9.1.6.
+
+::::
+
 ::::{dropdown} Alerts aren't generated for rules with alert flapping off and an alert delay higher than 1
 
 Applies to: {{stack}} 9.0.0-9.2.0
@@ -118,10 +133,10 @@ This was resolved in {{stack}} 9.0.3.
 
 Applies to: {{stack}} 9.0.0, 9.0.1, 9.0.2
 
-**Details** 
+**Details**
 Errors occur when rules run during an active maintenance window that has filters and a matching rule category. 
 
-**Workaround** 
+**Workaround**
 Remove any filters added to the active maintenance window.
 
 **Resolved**
@@ -167,7 +182,7 @@ Applies to: {{stack}} 9.0.0
 
 **Details**
 
-Rollup indices, like all indices, created in 7.x or earlier need to be reindexed in preparation for migration to 9.0. However, in addition to the normal reindex process the rollup job also needs to be accounted for. 
+Rollup indices, like all indices, created in 7.x or earlier need to be reindexed in preparation for migration to 9.0. However, in addition to the normal reindex process the rollup job also needs to be accounted for.
 
 **Action**