Add Milestone info to releases in Project Import/Export

What does this MR do and why?

This MR adds milestone information to releases when using Project Import/Export, so that when project is migrated, the milestone information is preserved.

Mentions #339422 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Create project with a milestone and release
2. Assign milestone to a release
3. Export project using Project Export functionality (under Settings -> Advanced)
4. Download exported tarball and import it back
5. Verify imported project has releases with milestone infomation preserved

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.