Flag issues and merge requests that are breaking or high-impact changes

Document a process for flagging MRs as disruptive changes, for both human visibility and as a "hook" for tooling.

• Is there a labeling process to follow for breaking change or some new label we can define?
• Can we automate the addition of labels on MRs when they are present on issues?
• What should the process be moving forward for flagging issues and MRs as potentially high-impact, breaking change, or may be of concern to customers?