Add support for CVE JSON 5.1 and CVE Services 2.3.0

[mprpic]

Review and update whatever is necessary based on the changes announced in:

• https://github.com/CVEProject/cve-services/releases/tag/v2.3.0
• https://github.com/CVEProject/cve-schema/releases/tag/v5.1.0-rc2 (still RC but is likely to be the final 5.1.0 release)