Add prefix to autocomplete path

What does this MR do?

Adds a prefix to a broken autocomplete URL to get it working again.

After the change by !34295 (merged) the global /autocomplete path no longer works and the /-/ (hyphen) prefix is required to avoid a 404

Fixes #332556 (closed)

Changes made here follows the path strings built for other auto-completes such as in:

• app/assets/javascripts/filtered_search/dropdown_user.js
• app/assets/javascripts/vue_shared/alert_details/components/sidebar/sidebar_assignees.vue
• app/assets/javascripts/projects/commits/store/actions.js
• ee/app/assets/javascripts/roadmap/mixins/filtered_search_mixin.js

Screenshots or Screencasts (strongly suggested)

See #332556 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.