feat: remove soft-deleted team visibility from milestones

### Deletions

* removed field `teamVisible: Boolean!` from `MilestoneType`
* removed field `roles: [String!]!` from `MilestoneType`
* removed parameter `visibleOnly: Boolean` from `teamMilestones` query
* removed parameter `visibleOnly: Boolean` from `milestones` Subscription

Closes #347