Colourise security summaries in merge requests

What does this MR do?

Colourise security summaries in merge requests

This adds bold and severity-specific colour styling to the numbers of critical and high severity vulnerabilities in the security-related merge request widgets.

This also adds the word "new" into the summaries.

Addresses #221084 (closed).

Screenshots

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