Stringify policy yaml response in scanExecutionPolicies graphql query

This MR updates the policy yaml response in
scanExecutionPolicies GraphQL query to stringify the keys of
the yaml. It used to be symbolised keys which will be difficult
to parse in the frontend.

EE: true
Changelog: changed

ee/app/graphql/resolvers/scan_execution_policy_resolver.rb

@@ -19,7 +19,7 @@ module Resolvers
           name: policy[:name],
           description: policy[:description],
           enabled: policy[:enabled],
-          yaml: policy.to_yaml,
+          yaml: YAML.dump(policy.deep_stringify_keys),
           updated_at: policy_configuration.policy_last_updated_at
         }
       end

ee/spec/graphql/resolvers/scan_execution_policy_resolver_spec.rb

@@ -57,7 +57,7 @@ RSpec.describe Resolvers::ScanExecutionPolicyResolver do
             name: 'Run DAST in every pipeline',
             description: 'This policy enforces to run DAST for every pipeline within the project',
             enabled: true,
-            yaml: policy.to_yaml,
+            yaml: YAML.dump(policy.deep_stringify_keys),
             updated_at: policy_last_updated_at
           }
         ]