Fix vulnerabilities related issues widget

What does this MR do and why?

Fix vulnerabilities related issues widget:

• Update widget title to "Related issues" as it was intended but didn't render because the slot was named wrong.
• Only render one loading icon inside the widget body.

I spotted these issues while using the RelatedIssuesBlock component in Add Related Issue widget to Metrics Details (gitlab-org/opstrace/opstrace#2901 - 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.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Vulnerabilities report

Before	After

Issues

No visual change.

Before	After

How to set up and validate locally

1. You need a GitLab Ultimate license.
2. Go to [GDK_HOST]/gitlab-org/gitlab-shell/-/security/vulnerability_report.
3. Click on + Submit vulnerability to add a new vulnerability.
4. Then while viewing the newly created vulnerability click on Create issue button.
5. Create the issue and navigate back to the newly created vulnerability.
6. Verify the changes.