Improve tooltips on collapsible milestone sidebar

Corresponding MRs

• CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17996

What does this MR do?

Adds the following tooltips for the Milestone view:

• Progress
• Start date
• Due date
• Issues
• Time tracking
  • Note: This is taken care of in the original MR
• Total issue weight
• Merge requests
• Reference

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

Confirm if tests are needed for these tooltips.

Why was this MR needed?

Requested by https://gitlab.com/gitlab-org/gitlab-ce/issues/25010.

Does this MR meet the acceptance criteria?

• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25010, specifically the Milestone section.