Include Incidents in Milestone dashboard

Proposal

A US Gov customer recently upgraded from GitLab 17.9.2-ee to 17.11.2-ee. They noticed that incident type issues are no longer included in the milestone dashboard, at one point they did show up on the dashboard. I was able to reproduce what is being experienced in 17.11.2 as well as 18.0.

The same thing happened with tasks, but they were recently brought back to the milestone dashboard in 18.0. We are proposing to restore incident type issues to the milestone dashboard.