Reduce On-Demand CI YAML raw string size

Uses the DAST template to avoid storing whole CI YAML in the db.

ee/app/services/ci/run_dast_scan_service.rb

@@ -11,15 +11,20 @@ module Ci
       full_scan_enabled: 'DAST_FULL_SCAN_ENABLED'
     }.freeze
 
-    def self.ci_template_raw
-      @ci_template_raw ||= Gitlab::Template::GitlabCiYmlTemplate.find('DAST').content
+    def self.ci_template
+      @ci_template ||= YAML.safe_load(ci_template_raw)
     end
 
-    def self.ci_template
-      @ci_template ||= YAML.safe_load(ci_template_raw).tap do |template|
-        template['stages'] = ['dast']
-        template['dast'].delete('rules')
-      end
+    def self.ci_template_raw
+      <<~YAML
+        include:
+        - template: DAST.gitlab-ci.yml
+        stages:
+        - dast
+        dast:
+          rules:
+            - when: always
+      YAML
     end
 
     def execute(branch:, **args)

ee/spec/services/ci/run_dast_scan_service_spec.rb

@@ -24,8 +24,13 @@ RSpec.describe Ci::RunDastScanService do
       expect(described_class.ci_template['stages']).to eq(['dast'])
     end
 
-    it 'has has no rules' do
-      expect(described_class.ci_template['dast']['rules']).to be_nil
+    it 'has one rule is always true' do
+      rules = described_class.ci_template['dast']['rules']
+
+      aggregate_failures do
+        expect(rules.size).to eq(1)
+        expect(rules).to include('when' => 'always')
+      end
     end
   end