Set target_branch to the ref branch when creating MR from issue

What does this MR do?

When creating a new MR from an issue page it sets the target_branch of the newly created merge request to the branch a user set as a source branch for the issue feature branch.

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

Why was this MR needed?

When creating a MR from the issue page a user can select a source branch which is used as the base for an issue feature branch. This branch should be set as target branch for the MR that is created at the same time. However it does not happen and default repository branch is always used instead as the target_branch.

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 #41727 (closed)