Add Serverless unavailable tag for the Configure APM Agent Central Configuration doc

Author: vishaangelova

solutions/observability/apm/apm-server/apm-agent-central-configuration.md

@@ -4,6 +4,7 @@ mapped_pages:
   - https://www.elastic.co/guide/en/observability/current/apm-configure-agent-config.html
 applies_to:
   stack: ga
+  serverless: unavailable
 products:
   - id: observability
   - id: apm