Add clarification about Elastic Agent upgrade on Serverless (#1720)

Added a clarification that Elastic Agents are not upgraded automatically

## Issue

Closes [#1792](elastic/ingest-docs#1792)

## Preview

https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1720/reference/fleet/fleet-agent-serverless-restrictions

Author: vishaangelova

reference/fleet/fleet-agent-serverless-restrictions.md

@@ -18,10 +18,15 @@ If you are using {{agent}} with [{{serverless-full}}](/deploy-manage/deploy/elas
 * The number of {{agents}} that may be connected to an {{serverless-full}} project is limited to 10 thousand.
 * The minimum supported version of {{agent}} supported for use with {{serverless-full}} is 8.11.0.
 
-$$$outputs-serverless-restrictions$$$
-**Outputs**
+### Outputs
 
-* On {{serverless-short}}, you can configure new {{es}} outputs to use a proxy, with the restriction that the output URL is fixed. Any new {{es}} outputs must use the default {{es}} host URL.
+On {{serverless-short}}, you can configure new {{es}} outputs to use a proxy, with the restriction that the output URL is fixed. Any new {{es}} outputs must use the default {{es}} host URL.
+
+### Upgrade
+
+{{agents}} are not automatically upgraded with the upgrade of the {{serverless-short}} project. You can upgrade standalone or {{fleet}}-managed {{agent}}s at your convenience.
+
+For more information, see [](upgrade-elastic-agent.md) and [](upgrade-standalone.md).
 
 
 ## {{fleet}} [fleet-serverless-restrictions]