Prevent N+1 queries when fetching issue references

What does this MR do and why?

Preload routes when fetching issue references via GraphQL

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Related to #517805 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Visit a group issue list (e.g. https://gitlab.com/groups/gitlab-org/-/issues)
2. Check the SQL queries for the (getIssuesEE GraphQL request)
3. Number of SQL queries should go down after this fix