Convert admin/settings controller specs to system specs (#31548)

spec/system/admin/settings/content_retention_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'Admin::Settings::ContentRetention' do
+  it 'Saves changes to content retention settings' do
+    sign_in admin_user
+    visit admin_settings_content_retention_path
+
+    fill_in media_cache_retention_period_field,
+            with: '2'
+
+    click_on submit_button
+
+    expect(page)
+      .to have_content(success_message)
+  end
+
+  def media_cache_retention_period_field
+    form_label 'form_admin_settings.media_cache_retention_period'
+  end
+end