Prepend blank line to close message on merge request

What does this MR do?

Adds a new line above the "Closes" message on a merge request.

Why was this MR needed?

Autoappended Closes to the end of merge request needs a new line in the beginning. When using descriptive commit messages and the commit message body is used for the body of the merge request the Closes message should appear on a line by itself.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#21710 (closed)