Update $red- variables to match GitLab UI

What does this MR do?

Updates the current $red- variable hex values to match those in GitLab UI.

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

N/A

Security

N/A

Closes #213824 (closed)