docs(agentless-integrations): document limit of 5 (elastic#1596)

Document that there is currently a limit of 5 agentless integrations per
project (i.e. ECH or Serverless project).

Related: elastic/kibana#218481


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<img width="856" alt="Screenshot 2025-06-04 at 10 25 41"
src="https://github.com/user-attachments/assets/c4050e60-654d-47da-90e6-586ee525402d"
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Co-authored-by: Brandon Morelli <[email protected]>

Author: andrewkroh

solutions/security/get-started/agentless-integrations.md

@@ -17,6 +17,7 @@ Agentless integrations provide a means to ingest data while avoiding the orchest
 
 ::::{important}
 There are currently no additional costs associated with deploying agentless integrations. 
+There is currently a limit of 5 agentless integrations per project. 
 ::::
 
 ## Generally available (GA) agentless integrations