Incoherent filtering by milestone in /projects/:id/issues and /groups/:id/issues endpoints

/projects/:id/issues and /groups/:id/issues API endpoints use different implementations to filter issues. /groups/:id/issues uses IssueFinderwhich seemingly has more capabilities.

One (exemplary) consequence is that passing "No Milestone" works for /groups/:id/issues but not for /projects/:id/issues. See #23194 (closed)