Add index to improve speed of vulnerability_reads

What does this MR do and why?

Add index on (project_id, state, report_type, severity, vulnerability_id) to speed up commonly executed queries.

Related to #355703 (closed)

Database review

Before

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/9197/commands/32619

After

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/9202/commands/32656

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.