Fix inconsistent usage of capitals in sidebar overview strings

Description

The commmit a3b93e17 introduced different strings for overview navigation links, to differentiate links to Project Overview, Group Overview and Subgroup Overview sites.

Unfortunately, the nouns after the Overview prefix does not have capitals whereas other links like Merge Request or Locked Files use capitals for all used nouns.

This MR updates all added string of a3b93e17 by replacing the first letter of the nouns by capitals.

Screenshots

Not needed.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

We are only changing some frontend strings which should not affect the quality/performance of the product. Tests will also be updated.

Security

We are only changing some frontend strings which does not need a security review.