Update maximum agents limit for serverless project to 30k (#7582)

## Summary

Updates the maximum agents limit for serverless project to 30k.

## Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [ ] Yes  
- [x] No  

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

Author: nimarezainia

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

@@ -15,7 +15,7 @@ products:
 
 If you are using {{agent}} with [{{serverless-full}}](/deploy-manage/deploy/elastic-cloud/serverless.md), note these differences from use with {{ech}} and self-managed {{es}}:
 
-* A maximum of 20,000 {{fleet}}-managed {{agents}} can be connected to an {{serverless-full}} project. This limit does not apply to standalone agents.
+* A maximum of 30,000 {{fleet}}-managed {{agents}} can be connected to an {{serverless-full}} project. This limit does not apply to standalone agents.
 * The maximum supported number of {{agent}} policies is 500.
 * The minimum supported version of {{agent}} supported for use with {{serverless-full}} is 8.11.0.