Update upcoming releases API docs
Problem to solve
Deduplicate content:
- Update https://docs.gitlab.com/ee/api/releases/#upcoming-releases and:
- Remove image
- Link to https://docs.gitlab.com/ee/user/project/releases/#schedule-a-future-release
- Update https://docs.gitlab.com/ee/user/project/releases/#schedule-a-future-release:
- Rename section to "Upcoming releases" to be on par with the API docs