Add total time spent to milestones

What does this MR do?

Adds total spent time on milestones

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

Why was this MR needed?

To quickly have an overview of the time spent on each 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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39335 (closed)