chore: enable pyupgrade fixes (#1806)

# What does this PR do?

The goal of this PR is code base modernization.

Schema reflection code needed a minor adjustment to handle UnionTypes
and collections.abc.AsyncIterator. (Both are preferred for latest Python
releases.)

Note to reviewers: almost all changes here are automatically generated
by pyupgrade. Some additional unused imports were cleaned up. The only
change worth of note can be found under `docs/openapi_generator` and
`llama_stack/strong_typing/schema.py` where reflection code was updated
to deal with "newer" types.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

llama_stack/providers/remote/safety/bedrock/bedrock.py

@@ -6,7 +6,7 @@
 
 import json
 import logging
-from typing import Any, Dict, List
+from typing import Any
 
 from llama_stack.apis.inference import Message
 from llama_stack.apis.safety import (
@@ -53,7 +53,7 @@ class BedrockSafetyAdapter(Safety, ShieldsProtocolPrivate):
             )
 
     async def run_shield(
-        self, shield_id: str, messages: List[Message], params: Dict[str, Any] = None
+        self, shield_id: str, messages: list[Message], params: dict[str, Any] = None
     ) -> RunShieldResponse:
         shield = await self.shield_store.get_shield(shield_id)
         if not shield: