UX improvements in MR diff settings dropdown
### Problem to solve
The dropdown from the ellipsis menu above the MR diff in the Changes tab doesn't show the current selection. Our [Pajamas guidelines for dropdowns](https://design.gitlab.com/components/dropdown#usage) state that with multi-select items such as this:
> Each item contains a checkbox and can be clicked to select that item. The dropdown stays open when making selections and each selection will send a request to update the content. The dropdown closes when a user clicks outside of the dropdown or if the user clicks the “x” icon in the top right corner.
This is an interesting use case as 2 items above the line are single-selection and open a new view, while the other item above the line and the 2 below the line are more along the lines of multi-select view preferences. As part of showing [Static Analysis findings in the MR diff](https://gitlab.com/gitlab-org/gitlab/-/issues/322689), we may want to utilize this component to show/ hide the finding. Let's discuss whether it's better UX to show a checkmark and static text in place of the text changing between show and hide. AFAIK most of our dropdowns use the :heavy_check_mark: pattern instead of changing the text (e.g. labels and assignees in issues and MRs).

Related to https://gitlab.com/gitlab-com/www-gitlab-com/-/issues/11204
cc @gitlab-com/gitlab-ux/ux-foundations
issue