fix: use conventional commit message to generate changelog

What does this MR do?

See https://docs.gitlab.com/ee/api/repositories.html#generate-changelog-data

The commit message defaults to "Add changelog for version X", this just adds a prefix for make it compliant with convention commits (see #594 (closed))

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines