UI: Update GitLab Duo feature capitalization

Need to update the UI to match these capitalizations:

• GitLab Duo AI Impact Dashboard
• GitLab Duo Chat
• GitLab Duo Code Explanation
• GitLab Duo Code Review Summary
• GitLab Duo Code Suggestions
• GitLab Duo Issue Description Generation
• GitLab Duo Issue Discussion Summary
• GitLab Duo Merge Commit Message Generation
• GitLab Duo Merge Request Summary
• GitLab Duo Product Analytics
• GitLab Duo Root Cause Analysis
• GitLab Duo Test Generation
• GitLab Duo Vulnerability Explanation
• GitLab Duo Vulnerability Resolution