Store rollout_milestone field in the milestones field in Stage entities

[DanielRyanSmith]

Almost all references to milestone fields are stored in Stage.milestones, which uses the MilestoneSet kind. The exception for this is rollout_milestone for enterprise features, which is a separate field from an older implementation with the milestone field unnecessarily separate from similar milestone references. Migrating this rollout_milestone field to instead live in the milestones field (as milestones.desktop_first), will consolidate this milestone field to be handled the same way all other milestone fields are.

[jrobbins]

Daniel addressed this in PR #3771.