Make `from` optional in the changelog API

What does this MR do?

The from argument in the changelog generation API is now optional. When unspecified, GitLab will use the tag of the release that came before the new release. This means that most users never have to specify the from argument, making it easier to generate changelogs for a new version.

This fixes gitlab-com/gl-infra/delivery#1542 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry Change to a feature not yet released (and which already has a changelog)
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content