Resolve "Add discussions check to automatic merge"

What does this MR do and why?

Adds the discussions state to the list of states that we allow the merge request to be in when allowing merge when checks pass to be set.

Behind a feature flag, and no frontend supported yet to test it locally, rather, we should ensure that a MR is still unmergeable when discussions are open when the feature flag is off, and that the auto merge strategy is not available when a MR has open discussions

Related to #420640 (closed)