Use `Issue#readable_by` to solve disparity in APIs for issue links

What does this MR do and why?

When the issue of a vulnerability issue link belongs to another project and the issues feature is disabled for the project holding the vulnerability, the issue link can not be read through GraphQL API.

This MR uses an existing method of issue model to solve this problem.

Note: We can't change the delegate to the issue's project as the admin_vulnerability_issue_link rule won't work.

Related to GraphQL Vulnerability query not returning issue... (#408917 - closed).

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.