Create merge request: Consider adding labels to identify source | target project and branch

Description

A user identified a situation where buttons in our Create merge request flow were not announcing their accessible labels correctly in macOS Chrome + VoiceOver. While root-causing that issue, I noticed the dropdowns for Source and Target could be improved by adding labels to the Project and Branch dropdowns.

Screen reader users especially would benefit from hearing a label like "Source project, cloned-gitlab.org", then "Source branch, main", and so on for the target project and branch.

We have such a pattern already in place in Pajamas:

• Design guidance for externally labeled listboxes
• Storybook example of a searchable listbox with label

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