Make issue vulnerability popover less subtle

What does this MR do and why?

It introduces some UI changes to the popover that shows related issue for a vulnerability within the vulnerability list:

• Changes icons
• Adds "(closed)" text to closed issues
• Increases spacing between the icon and the text

Screenshots or screen recordings

before	after

How to set up and validate locally

If you already have a working runner, skip to the bullet-list below.

You should have the runner installed in order to run the pipeline. Then using the following command you can register the runner, it guides you pretty good on how to set the runner:

$ gitlab-runner register

Once it's installed this is how I run the runner:

$ gitlab-runner --log-level debug run local-runner --config ~/.gitlab-runner/config.toml restart

After you got the runner running:

1. Clone https://gitlab.com/gitlab-examples/security/security-reports/
2. Run the pipeline by going into Your project > CI/CD > Pipelines
3. Click on run pipeline for master branch
4. Go to Security & Compliance > Vulnerability Report
5. Click on a vulnerability
6. Create at least 3 new issues for the vulnerability
7. Go back to the Vulnerability Report and you can see the related-issues badge with the pop-over

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.

Related to #332901 (closed)