Update `new_milestone_path` from `url` to `path`

What does this MR do?

Updates the value of new_milestone_path passed to the frontend on the Edit Release page to be a relative path (generated using new_project_milestone_path) instead of an absolute path (new_project_milestone_url). This is consistent with the other paths passed to this page.

This update makes no visual changes to this page.

Context

This change was recommended by @dzaporozhets in this comment.

Screenshots

This path is used to generate this link: