Issue milestone remaining time tooltip

What does this MR do?

This MR fixes the bug outlined in #23493 (closed). This MR introduces a new method "remaining_time" to return the correct days/months/years remaining until the milestone is due. This MR also adds additional specs and optimisations for the milestone_remaining_days and remaining_days methods.

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

There is nothing obvious that needs to be checked.

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• Changelog entry added
• 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?

#23493 (closed)