fix(event-handler): fix ruff lint violations in graphql_appsync/_registry.py docstrings

Update docstring return type hints to use modern Python 3.10+ syntax:
- Optional[Dict] -> dict | None in Returns section

Part of #8088

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

Author: hirenkumar-n-dholariya

aws_lambda_powertools/event_handler/graphql_appsync/_registry.py

@@ -64,7 +64,7 @@ def find_resolver(self, type_name: str, field_name: str) -> dict | None:
             Field name
         Return
         ----------
-        Optional[Dict]
+        dict | None
             A dictionary with the resolver and if raise exception on error
         """
         logger.debug(f"Looking for resolver for type={type_name}, field={field_name}.")