Use lazy association loader for issue links

What does this MR do and why?

This MR changes the way we preload the issue links for vulnerabilities on GraphQL by using the lazy relation loader introduced by !115240 (merged).

The new logic is behind a feature flag to make sure we can easily switch back to previous version of the logic if something goes wrong.

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.