fix(event-handler): fix ruff lint violations in appsync.py docstrings

Update docstring parameter type hints to use modern Python 3.10+ syntax:
- Optional[str] -> str | None in field_name parameter descriptions

Part of #8088

Signed-off-by: hirenkumar-n-dholariya <hirenkumarnd@gmail.com>

Author: hirenkumar-n-dholariya

aws_lambda_powertools/event_handler/appsync.py

@@ -392,7 +392,7 @@ def resolver(self, type_name: str = "*", field_name: str | None = None) -> Calla
         ----------
         type_name : str, optional
             GraphQL type e.g., Query, Mutation, by default "*" meaning any
-        field_name : Optional[str], optional
+        field_name : str | None, optional
             GraphQL field e.g., getTodo, createTodo, by default None
 
         Returns
@@ -447,7 +447,7 @@ def batch_resolver(
         ----------
         type_name : str, optional
             GraphQL type e.g., Query, Mutation, by default "*" meaning any
-        field_name : Optional[str], optional
+        field_name : str | None, optional
             GraphQL field e.g., getTodo, createTodo, by default None
         raise_on_error : bool, optional
             Whether to fail entire batch upon error, or handle errors gracefully (None), by default False