Use different feature flag for epic issue adapter

Since `work_item_epics_ssot` is already used for other parts of the
code, it's safer to use a separate feature flag for this rollout.

ee/app/services/work_items/legacy_epics/epic_issues/create_service.rb

@@ -13,7 +13,7 @@ def initialize(legacy_epic, user, params)
         end
 
         def execute
-          if legacy_epic.group.work_item_epics_ssot_enabled?
+          if ::Feature.enabled?(:epic_issues_through_work_item_service, legacy_epic.group)
             @existing_epic_issue_ids = EpicIssue.in_epic(legacy_epic.id)
               .for_issue(referenced_child_work_items.map(&:id)).pluck_primary_key
 

ee/config/feature_flags/gitlab_com_derisk/epic_issues_through_work_item_service.yml

+---
+name: epic_issues_through_work_item_service
+feature_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/504273
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179819
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/516175
+milestone: '17.9'
+group: group::product planning
+type: gitlab_com_derisk
+default_enabled: false

ee/spec/services/work_items/legacy_epics/epic_issues/create_service_spec.rb

@@ -123,9 +123,9 @@ def assign_issue(references)
       context 'when user has permissions to link the issue' do
         let_it_be(:user) { guest }
 
-        context 'when work_item_epics_ssot is disabled' do
+        context 'when epic_issues_through_work_item_service is disabled' do
           before do
-            stub_feature_flags(work_item_epics_ssot: false)
+            stub_feature_flags(epic_issues_through_work_item_service: false)
           end
 
           it 'calls the legacy service' do