BulkImports: Import milestone `iid`

Problem

Currently the BulkImports::Groups::Pipelines::MilestonesPipeline is not importing the milestone iid field, because it is not exposed in the GraphQL API.

Proposed solution

After exposing the iid on the GraphQL API (#326129 (closed)), add it to the query in BulkImports::Groups::Graphql::GetMilestonesQuery