#13336 - display multiple messages in both the UI and git output

What does this MR do?

This modifies the UI to display all current broadcast messages rather than just one.

The message returned to git clients is similarly updated.

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

Is there anything else I need to update here?

Why was this MR needed?

For feature proposal #13336 (closed) - as things stands broadcast messages can be missed if there are multiple active ones

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #13336 (closed)