Update Source Repo dropdown to be Text

@sming-gitlab Good catch on the source project being disabled since there can only be that option. I noticed the new merge request handles this with just has a dropdown with 1 option. Technically if there is only one option we should not be using a dropdown per https://design.gitlab.com/components/dropdown#usage. I wanted to get your thoughts on just having text from both a feasibility and a how do you feel about it perspective.