Present only issues visible to user in Vulnerabilitiy Issue Links API

What does this MR do?

Related to #9424 (closed)

As discovered in #9424 (comment 379950936) we have noticed that current API is presenting basic information about issues that are linked to selected Vulnerability, but not visible to the user. In current release we have changed API to allow to link Vulnerability with Issues from other projects, this change improves problem with visibility.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team