Improve visual consistency of values in vulnerability modal

What does this MR do?

Severity values are now rendered using the <severity-badge> component, to match how they're displayed in the vulnerability list.

Confidence values are now also capitalised, for the same reason.

Before:

After:

What are the relevant issue numbers?

#9117 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• EE specific content should be in the top level /ee folder
• Security reports checked/validated by reviewer

Closes #9117 (closed)