Add Serverless unavailable tag for the Configure APM Agent Central Configuration doc (elastic#3371)

This PR updates the page-level `applies_to` for the [Configure APM Agent
Central
Configuration](https://www.elastic.co/docs/solutions/observability/apm/apm-server/apm-agent-central-configuration)
doc to include a “Serverless unavailable” tag.

Resolves elastic#3367

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