Update real-user-monitoring-rum.md (#3886)

Add clarification that APM RUM is not available for Serverless.

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Co-authored-by: Visha Angelova <91186315+vishaangelova@users.noreply.github.com>

Author: maggieghamry

solutions/observability/apm/apm-agents/real-user-monitoring-rum.md

@@ -3,6 +3,7 @@ mapped_pages:
   - https://www.elastic.co/guide/en/observability/current/apm-rum.html
 applies_to:
   stack: all
+  serverless: unavailable
 products:
   - id: observability
   - id: apm