Show single count for severity group

What does this MR do and why?

Simply display counts instead of full text next to severity categories.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable :vulnerability_report_grouping feature flag
2. Go to Vulnerability Report
3. Click on Group By: Severity
4. Check the counts

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #426674 (closed)