[Obs AI] Update Agent Builder note (#6307)

## Summary
Updates to reflect Agent Builder becoming the default chat experience.

- Updates obs Agent Builder note
- Move Agent Builder up in TOC

Author: mdbirnstiehl

solutions/observability/ai/agent-builder-observability.md

@@ -16,8 +16,8 @@ description: Learn how Elastic Agent Builder works with Elastic Observability
 Agent Builder integrates tightly with {{observability}}, shipping with built-in agents and tools designed for observability use cases, and you can create your own custom agents and tools to fit your specific needs. Combine your agents with [Elastic Workflows](/explore-analyze/workflows.md) to automatically isolate hosts, create cases, send notification messages to external platforms, and more.
 
 :::{note}
-:applies_to: {stack: preview =9.3}
-In {{stack}} version 9.3, you need to [opt in](/explore-analyze/ai-features/ai-chat-experiences/ai-agent-or-ai-assistant.md) to use Agent Builder in {{observability}}.
+- {applies_to}`stack: ga 9.4+` {applies_to}`serverless: ga` {{agent-builder}} is the default chat experience in {{observability}}.
+- {applies_to}`stack: preview =9.3` In {{stack}} version 9.3, you need to [opt in](/explore-analyze/ai-features/ai-chat-experiences/ai-agent-or-ai-assistant.md) to use Agent Builder in {{observability}}.
 :::
 
 ## Recommended models

solutions/observability/ai/observability-ai-assistant.md

@@ -14,7 +14,7 @@ products:
 
 :::{warning}
 :applies_to: {"stack": "ga 9.4", "serverless": "ga"}
-The {{obs-ai-assistant}} is deprecated. The [Elastic AI Agent](/explore-analyze/ai-features/elastic-agent-builder.md) is now the default chat experience in {{observability}}. To switch back to the AI Assistant, go to **GenAI settings**.
+The {{obs-ai-assistant}} is deprecated. The [Elastic AI Agent](/explore-analyze/ai-features/elastic-agent-builder.md) is now the default chat experience. To switch back to the AI Assistant, go to **GenAI settings**. Refer to [Compare Agent Builder and AI Assistant](/explore-analyze/ai-features/ai-chat-experiences/ai-agent-or-ai-assistant.md) for more information.
 :::
 
 The AI Assistant is an integration with a large language model (LLM) that helps you understand, analyze, and interact with your Elastic data.

solutions/toc.yml

@@ -526,8 +526,8 @@ toc:
       - file: observability/data-set-quality-monitoring.md
       - file: observability/ai/observability-ai.md
         children:
-          - file: observability/ai/observability-ai-assistant.md
           - file: observability/ai/agent-builder-observability.md
+          - file: observability/ai/observability-ai-assistant.md
           - file: observability/ai/automatic-import.md
           - file: observability/ai/llm-performance-matrix.md
       - file: observability/cross-project-search.md