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? #429009.

Proposal

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