Fix milestones path from issues sidebar

What does this MR do?

In sidebar it sets the correct link for milestones:

• if a milestone is a group milestone the link leads to the group milestone
• if a milestone is a project milestone the link leads to the project milestone

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The milestone link in the sidebar always led to a project milestone

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #35481 (closed)