Keep subscribers when promoting labels to group labels

What does this MR do?

When a project label is promoted to a group label it keeps users' subscriptions.

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

Why was this MR needed?

The subscriptions are not kept now when a label is promoted.

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
• Internationalization required/considered

What are the relevant issue numbers?

Closes #37199 (closed)