Should use "MEDIUM" instead of "MODERATE" in CVSS rating

[sschuberth]

While the docs seem to state that both "MEDIUM" and "MODERATE" would be valid CVSS ratings as the docs are linking to "Common Vulnerability Scoring System (CVSS), Section 5", that's actually not true: The CVSS specs only mention "MEDIUM" as a valid value. As such, the GitHub Advisories API should also only return "MEDIUM" instead of "MODERATE".