Use API release code and remove Git based release code
This removes the old Git based release code in favour the API release code. This means we now always perform our self-managed releases using the API.
This closes gitlab-com/gl-infra/delivery#1226 (closed), gitlab-com/gl-infra/delivery#1148 (closed), gitlab-com/gl-infra/delivery#1216 (closed), gitlab-com/gl-infra/delivery#712 (closed)
Edited by Yorick Peterse