Update security dashboard vulnerability filter styling

What does this MR do?

On the security dashboard, there are filters that lets the user filter the vulnerability list:

This MR updates the filter dropdown style from the deprecated dropdown to the current GitLab UI dropdown.

Before	After

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

Relates to #229661 (closed)