Dependabot now supports alerting on artifacts and non-default branches [GA]

### Value Prop
Dependabot can now generate security alerts for artifacts—such as containers, packages, and executables—as well as dependencies on non-default branches. By uploading a software bill of materials (SBOM) to GitHub's snapshot service and flagging it as alert-worthy, teams gain continuous vulnerability monitoring for every released artifact and long-running branch, not just the default branch. This closes a critical blind spot where production services and shipped products could be silently exposed to newly discovered vulnerabilities in pinned dependency versions.

### Expected Outcome
Non-default branch alerting is the most highly requested Dependabot feature in customer feedback. With this release, customers can detect zero-day vulnerabilities in any released artifact or long-lived branch—ensuring that the software they ship and operate stays protected even after the repository has moved on. We expect this to significantly reduce unpatched exposure windows, strengthen end-to-end supply chain security, and reinforce GitHub as the trusted platform for securing the entire software delivery pipeline.