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/utils/kvstore/api.py

@@ -5,15 +5,15 @@
 # the root directory of this source tree.
 
 from datetime import datetime
-from typing import List, Optional, Protocol
+from typing import Protocol
 
 
 class KVStore(Protocol):
     # TODO: make the value type bytes instead of str
-    async def set(self, key: str, value: str, expiration: Optional[datetime] = None) -> None: ...
+    async def set(self, key: str, value: str, expiration: datetime | None = None) -> None: ...
 
-    async def get(self, key: str) -> Optional[str]: ...
+    async def get(self, key: str) -> str | None: ...
 
     async def delete(self, key: str) -> None: ...
 
-    async def range(self, start_key: str, end_key: str) -> List[str]: ...
+    async def range(self, start_key: str, end_key: str) -> list[str]: ...