Make `to` in the changelog API optional

What does this MR do?

The to attribute now defaults to the target branch itself. This means that for most projects one only needs to specify the version of a new release, simplifying the use of the API.

This fixes gitlab-com/gl-infra/delivery#1556 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry Feature is being released in 13.9, and a changelog was previously added
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content