Display Merge Request's source branch name in sidebar

What does this MR do?

Adds "Branch: " to the merge request sidebar, below "Reference: ...".

Closes #23767 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers