refactor: move all datetime.now() calls to UTC (#1589)

# What does this PR do?

Updated all instances of datetime.now() to use timezone.utc for
consistency in handling time across different systems. This ensures that
timestamps are always in Coordinated Universal Time (UTC), avoiding
issues with time zone discrepancies and promoting uniformity in
time-related data.

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/inline/agents/meta_reference/persistence.py

@@ -7,7 +7,7 @@
 import json
 import logging
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import List, Optional
 
 from pydantic import BaseModel
@@ -36,7 +36,7 @@ class AgentPersistence:
         session_info = AgentSessionInfo(
             session_id=session_id,
             session_name=name,
-            started_at=datetime.now(),
+            started_at=datetime.now(timezone.utc),
         )
         await self.kvstore.set(
             key=f"session:{self.agent_id}:{session_id}",