Add prefix to autocomplete path

After the change by
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34295
the global /autocomplete path no longer works
and the /-/ (hyphen) prefix is required to avoid a 404

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/332556

Changelog: fixed

app/assets/javascripts/filtered_search/available_dropdown_mappings.js

@@ -204,7 +204,7 @@ export default class AvailableDropdownMappings {
   getMergeRequestTargetBranchesEndpoint() {
     const endpoint = `${
       gon.relative_url_root || ''
-    }/autocomplete/merge_request_target_branches.json`;
+    }/-/autocomplete/merge_request_target_branches.json`;
 
     const params = {
       group_id: this.getGroupId(),