Add Known issues section with an issue about Functionbeat (#20859)

CHANGELOG.asciidoc

@@ -229,6 +229,12 @@ field. You can revert this change by configuring tags for the module and omittin
 - Deprecate tags config parameter in cloudwatch metricset. {pull}16733[16733]
 - Deprecate tags.resource_type_filter config parameter and replace with resource_type. {pull}19688[19688]
 
+==== Known Issues
+
+*Functionbeat*
+
+- Functionbeat cannot be deployed on Google Cloud Platform. {issue}20830[20830]
+
 [[release-notes-7.8.1]]
 === Beats version 7.8.1
 https://github.com/elastic/beats/compare/v7.8.0...v7.8.1[View commits]