Replace Test Coverage Visualization performance issue link

What does this MR do?

This merge request replaces the generic jUnit performance issue link (#37725 (closed)) with the more specific Test Coverage Visualization one (#211410 (closed)). The Enabling the feature chapter in the Test Coverage Visualization documentation references an issue, which has to be closed before the feature reaches GA. I initially used the generic jUnit performance issue in the chapter, which has been superset by #211410 (closed). This change should make the required actions clearer and prevent confusion, since people would see a closed jUnit issue.

documentation

/cc @marcel.amirault @rickywiens

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