[NavGroup] Apply font opacity instead of color

[MarvinKlein1508]

Pull Request

📖 Description

This PR removes the second color for disabled FluentNavGroup elements and it applies the same opacity approach from FluentNavItem to the FluentNavGroup

Before:

After:

🎫 Issues

Follow up from #4647

👩‍💻 Reviewer Notes

📑 Test Plan

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps