Revise the navbar to use sentence case

What does this MR do?

Modifies yet another instance of improperly capitalized Merge Request, Value Stream, Code Review, DevOps Adoption, and the items around it for consistency.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #233449 (closed)