[BE] Add Reachability status to GraphQL for Vulnerability Report

Reference: [BE] Expose Reachability status to Vulnerabilit... (#513990 - closed)

"Reachable" needs to be made available in all 3 pages:

• https://docs.gitlab.com/ee/api/graphql/reference/#queryvulnerabilities
• https://docs.gitlab.com/ee/api/graphql/reference/#groupvulnerabilities
• https://docs.gitlab.com/ee/api/graphql/reference/#projectvulnerabilities

Query.vulnerabilities
Project.vulnerabilities
Group.vulnerabilities