Fixes #22622: Include MR times in Milestone time overview

What does this MR do?

In #22622 (closed) it is described that the milestone time overview should include the time recorded in the related milestones as well. I agree with this, so this MR tries to fix this. I've mostly based this on the work done in !16753, and adjusted the implementation to add the MR times as well.

I do not know how to test this locally, but I assume that if the tests succeed, this change should work just fine. Next to that, this is my first ever Ruby MR, so any feedback is welcome.

Screenshots

I have none.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

This change should not impact availability. Relevant tests have been updated.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #22622 (closed)