Disable UI and document limitation with divergent fast-forward or semi-linear trains
We won't be supporting skip-merges for semi-linear / fast-forward merge trains right away.
Document limitation and update `merge_trains_skip_train_allowed?` https://gitlab.com/gitlab-org/gitlab/-/issues/429009.
### Proposal
```ruby
def merge_trains_skip_train_allowed?
merge_trains_skip_train_allowed &&
merge_trains_enabled? &&
!project.ff_merge_must_be_possible? && # Not yet supported, see https://gitlab.com/gitlab-org/gitlab/-/issues/429009
::Feature.enabled?(:merge_trains_skip_train, project)
end
```
issue