Update create-upload-source-maps-rum.md (#3887)

Add an update to clarify that source maps (RUM) is not available for
Serverless projects

---------

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

Author: maggieghamry

solutions/observability/apm/apm-agents/create-upload-source-maps-rum.md

@@ -3,6 +3,7 @@ mapped_pages:
   - https://www.elastic.co/guide/en/observability/current/apm-source-map-how-to.html
 applies_to:
   stack: ga
+  serverless: unavailable
 products:
   - id: observability
   - id: apm
@@ -119,7 +120,7 @@ curl -X POST "http://localhost:5601/api/apm/sourcemaps" \
 ```
 
 1. This example uses the version from `package.json`
-2. The API key used here needs to have appropriate privileges. Refer to the [{{stack}}](https://www.elastic.co/docs/api/doc/kibana/group/endpoint-apm-sourcemaps) or [{{serverless-short}}](https://www.elastic.co/docs/api/doc/serverless/group/endpoint-apm-sourcemaps) API documentation.
+2. The API key used here needs to have appropriate privileges. Refer to the [{{stack}}](https://www.elastic.co/docs/api/doc/kibana/group/endpoint-apm-sourcemaps) API documentation.
 
 ### Upload via a custom app [apm-source-map-custom-app]