Change "Todo" to "To Do" in UI

What does this MR do?

Changes "Todo" to "To Do" in the UI for clarity. Related CE issue is here: https://gitlab.com/gitlab-org/gitlab-ce/issues/61501

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

This change should have no impact on performance or testing.

Security

This change should have no impact on security.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61501