Use defauly `only` policy defined as a constant

We reuse this constant in EE.

lib/gitlab/ci/config/entry/job.rb

@@ -67,7 +67,7 @@ module Gitlab
 
           entry :only, Entry::Policy,
             description: 'Refs policy this job will be executed for.',
-            default: { refs: %w[branches tags] }
+            default: Entry::Policy::DEFAULT_ONLY
 
           entry :except, Entry::Policy,
             description: 'Refs policy this job will be executed for.'

lib/gitlab/ci/config/entry/policy.rb

@@ -11,6 +11,8 @@ module Gitlab
           strategy :RefsPolicy, if: -> (config) { config.is_a?(Array) }
           strategy :ComplexPolicy, if: -> (config) { config.is_a?(Hash) }
 
+          DEFAULT_ONLY = { refs: %w[branches tags] }.freeze
+
           class RefsPolicy < ::Gitlab::Config::Entry::Node
             include ::Gitlab::Config::Entry::Validatable