Fix focus-visibility for vulnerability actions

What does this MR do?

When navigating through the security dashboard using the keyboard, the action buttons for a given vulnerability stay hidden, even when they have focus.

This MR changes the responsible CSS-selector from using :focus to :focus-within so the row of buttons is set to be visible whenever an element inside has focus.

Note: The pseudo-selector is not supported by IE and only recent versions of Edge.

Before

After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team