Menu items: fix capitalization of Merge Request

This merge request is part of the sequence for #233449 (closed) that is gradually fixing all the miscapitalized instances of "Merge Request" in our UI. Because I'm unfamiliar with FE testing, I'm landing a set of MVCs that gradually change the UI, rather than cleaning up all issues at once.

Related issues

Related to #233449 (closed) where I'm working through the entire UI