Refactor to use listbox-item in MR add approval rule drawer

Issue

the target branch dropdown in MR add approval rule drawer only been partially migrated. The bottom two actions seem to be using the old styles (hover background should not span the whole width of the dropdown).

Desired behaviour:

should use listbox-item instead of deprecated dropdown components.

Implementation notes:

update <gl-dropdown-item/> in ee/app/assets/javascripts/vue_shared/components/branches_selector/protected_branches_selector.vue (ref) to use <gl-listbox-item/>. refer to gitlab-ui source code for component API.