Convert newline symbols in description push options to actual newlines

What does this MR do and why?

This MR converts literal '\n' symbols passed via the merge_request.description push option into actual newlines.

Related to Push Options don't allow newlines in values (#241710 - closed)

Screenshots or screen recordings

console	MR view	MR edit

How to set up and validate locally

1. Clone a repository
2. Create a branch
3. Make a change and commit
4. Push branch with a push option -o mr.create and -o mr.description

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.