Move epic type filter behind feature flag

What does this MR do and why?

Move the issue list epic type filter behind the namespace_level_work_items feature flag, which controls when epics can appear in the list.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Open a group issues page with :work_items_mvc_2 disabled
2. In the filtered search, use the Type filter and observe the options available do not include "Epic"
3. Enable :work_items_mvc_2 and verify the Epic filter appears

app/controllers/groups_controller.rb

@@ -41,6 +41,7 @@ class GroupsController < Groups::ApplicationController
     push_force_frontend_feature_flag(:work_items_beta, group.work_items_beta_feature_flag_enabled?)
     push_force_frontend_feature_flag(:work_items_mvc_2, group.work_items_mvc_2_feature_flag_enabled?)
     push_force_frontend_feature_flag(:linked_work_items, group.linked_work_items_feature_flag_enabled?)
+    push_frontend_feature_flag(:namespace_level_work_items, group)
     push_frontend_feature_flag(:issues_grid_view)
     push_frontend_feature_flag(:group_multi_select_tokens, group)
   end