Refactor issues filter in API

What does this MR do?

Ruse the IssueFinder for finding and filtering issues in the API. We cannot use the label filter of the issue finder because it matches only if all labels matches. The current API however matches if any of the label matches. I propose to change this in 9.0 since the /groups/:id/issues already uses the issue finder for that.

lib/api/issues.rb

@@ -16,6 +16,7 @@ def filter_issues_labels(issues, labels)
                             desc: 'Return issues ordered by `created_at` or `updated_at` fields.'
         optional :sort, type: String, values: %w[asc desc], default: 'desc',
                         desc: 'Return issues sorted in `asc` or `desc` order.'
+        optional :milestone, type: String, desc: 'Return issues for a specific milestone'
         use :pagination
       end