Resolve "GitLab Migration - Migrate Issue Boards"

What does this MR do and why?

Add issue boards that belong to projects to the import migration tool (behind the :bulk_import_projects feature flag)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

1. Feature.enable(:bulk_import)
2. Feature.enable(:bulk_import_projects)
3. Create a top level group and project in it
4. Create an issue board in the new project
5. Go to /groups/new#import-group-pane page and enter instance url and access token (needs to be api and read_repository scope)
6. Select newly created group and click Import
7. Wait for Group import to complete and verify imported project's issue board and issue lists

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #339418 (closed)