Add fork error when the user already forked the project
What should be the behaviour when the user already forked the project in the [new fork form](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53544)?
This is the old way:

This functionality was a requirement that existed in our old testing spec file > https://gitlab.com/gitlab-org/gitlab/-/blob/a8063e249382969ac1ddddcd2ecef88a6f0265d4/spec/features/projects/fork_spec.rb#L233
issue