Use REST API instead of GraphQL #401
Description
The tool uses GitHub's GraphQL API for getting the alerts.
As I remember it, the alerts weren't available via the REST API when we built it.
But it is now: https://docs.github.com/en/rest/dependabot/alerts#list-dependabot-alerts-for-a-repository
Maybe we can provide better error messages when using the REST API (see #138).