remove durable_handler from code-base

[yaythomas]

overview

The original task #97 was to rename durable_handler to durable_execution.

https://github.com/aws/aws-durable-execution-sdk-python/blob/main/src/aws_durable_execution_sdk_python/execution.py#L190

There is now a durable_execution decorator, but durable_handler is still there. We should remove this before launch to avoid confusing consumers.

acceptance criteria

1. remove durable_handler.
2. Update testing repo that still references durable_handler to use durable_execution instead.
3. Update specification