Resolve "Import with Milestones Failing"

This MR is a refactor that fixes a few things to do with Milestones and Labels.

In general, it reuses any existing group labels. If the group or project labels do not exist, it always imports them all at project level.

Milestone IIDs are preserved, however in the case of a Group Milestone, if it can't find an existing one, it will create a new one with the next available IID. If this IID is later on used by one of the fixed Project Milestones, then it will replace the former IID and assign a new one to the group milestone.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43270 (closed)

Closes #47626 (closed)