Replace spinner with skeleton loader in explain vulnerability drawer

What does this MR do and why?

Replace spinner with skeleton loader in explain vulnerability drawer

Note: Other UI updates are in Update vulnerability drawer to match others (!121873 - merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the openai_experimentation, ai_experimentation_api, and explain_vulnerability feature flags.
2. Clone this project: https://gitlab.com/gitlab-examples/security/security-reports (or use any project with a SAST vulnerability)
3. Run a pipeline against the master branch.
4. Go to Security -> Vulnerability report.
5. Click on a SAST vulnerability to go to the details page.
6. Click on the "Explain this vulnerability" button.

#411677 (closed)