Enable RuboCop rule `Performance/OpenStruct`

Exclude current offenses in `.rubocop_manual_todo.yml` and link the
follow-up issue.

.rubocop.yml

@@ -640,3 +640,7 @@ Cop/UserAdmin:
     - 'lib/gitlab/auth/current_user_mode.rb'
     - 'spec/**/*.rb'
     - 'ee/spec/**/*.rb'
+
+Performance/OpenStruct:
+  Exclude:
+    - 'ee/spec/**/*.rb'

.rubocop_manual_todo.yml

@@ -2573,3 +2573,20 @@ Cop/UserAdmin:
     - 'lib/gitlab/performance_bar.rb'
     - 'lib/gitlab/visibility_level.rb'
     - 'qa/qa/runtime/api/client.rb'
+
+# WIP https://gitlab.com/gitlab-org/gitlab/-/issues/325744
+Performance/OpenStruct:
+  Exclude:
+    - 'Guardfile'
+    - 'app/finders/snippets_finder.rb'
+    - 'app/helpers/application_settings_helper.rb'
+    - 'app/models/cycle_analytics/project_level_stage_adapter.rb'
+    - 'ee/lib/gitlab/graphql/aggregations/epics/epic_node.rb'
+    - 'ee/lib/gitlab/graphql/aggregations/epics/epic_node.rb'
+    - 'lib/api/wikis.rb'
+    - 'lib/gitlab/ci/ansi2html.rb'
+    - 'lib/gitlab/ci/reports/test_suite_comparer.rb'
+    - 'lib/gitlab/git/diff_collection.rb'
+    - 'lib/gitlab/import_export/after_export_strategies/base_after_export_strategy.rb'
+    - 'lib/gitlab/testing/request_inspector_middleware.rb'
+    - 'lib/mattermost/session.rb'

.rubocop_todo.yml

@@ -323,10 +323,6 @@ Performance/Detect:
 Performance/MethodObjectAsBlock:
   Enabled: false
 
-# Offense count: 42
-Performance/OpenStruct:
-  Enabled: false
-
 # Offense count: 18
 # Cop supports --auto-correct.
 # Configuration parameters: AutoCorrect.