31885 - Ability to transfer a single group to another group

What does this MR do?

Ability to transfer a single group to another group

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

• General structure of the new service
• Double check that the service don't leave any loose end

Why was this MR needed?

It's useful to allow transferring a group either as a subgroup or as a root group

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #31885 (closed)

To do

• Readjust UI
• Finish the specs
• Use one single service for both scenarios: Transferring a group into a subgroup and transforming a subgroup into a root
• Restore old visibility in descendants in case of something goes wrong when transferring a group
• Refactor
• Readjust changelog
• Remove js.erb response