Draft: Move explain vulnerability button

What does this MR do and why?

Per the latest designs, the "Explain vulnerability" button should be separate and not part of the split-button.

This MR adds the UI change that moves the button.

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

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Relates to: https://gitlab.com/gitlab-org/gitlab/-/issues/473770