Use `aws-actions/setup-sam` to set up

The sample provides a bit dated code. I think using `aws-actions/setup-sam@v2` would remove some boilerplate, and `actions/checkout@v2` is also a bit dated - should be `v4` at this point.

Using `setup-sam` would also imply having a `samconfig.toml` in the root.

Start by reading the repository's GitHub Actions workflow and the deployment configuration used by the sample. Update the workflow to use aws-actions/setup-sam@v2 and actions/checkout@v4, add the requested root-level samconfig.toml, and verify that the continuous delivery example still works.

Written by the indexing model from the issue text.