Update branch selector component in new merge request creation flow

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Context

In #518021 (closed), the branch selector was updated to allow users to:

• View the default branch at the top of the list
• Allow users to quickly copy the ref name
• More easily view long branch names with a longer branch selector width

The design solution resolved #518021 (closed), #346777 (closed), and #438154. This updated branch selector has not yet been percolated to the flow when users create a new merge request.

Current MR branch selector in new MR flow	New Source Code branch selector

@nickleonard

this is in the MR creation flow but I don't know which in this project is the default branch — there's main and master 

Design proposal

In the MR creation flow (Create merge request > Change branches > Select Source or Target branch selector):

• Use the new branch selector component implemented in #518021 (closed)
• If needed, make sure the default label appears on the default branch. I suspect this will be resolved if we use the correct component.