Use push method on Auditable module

External collaborators to `Gitlab::Audit::EventQueue` should not know
about the internals of the queue. Hence, we should use a public method,
instead of interacting with the array using `<<`.

ee/app/models/concerns/auditable.rb

@@ -4,6 +4,6 @@ module Auditable
   def push_audit_event(event)
     return unless ::Gitlab::Audit::EventQueue.active?
 
-    ::Gitlab::Audit::EventQueue.current << event
+    ::Gitlab::Audit::EventQueue.push(event)
   end
 end

ee/spec/lib/gitlab/audit/auditor_spec.rb

@@ -13,8 +13,8 @@ RSpec.describe Gitlab::Audit::Auditor do
   let(:remove_message) { 'Removed an interesting field from project Gotham' }
   let(:operation) do
     proc do
-      ::Gitlab::Audit::EventQueue.current << add_message
-      ::Gitlab::Audit::EventQueue.current << remove_message
+      ::Gitlab::Audit::EventQueue.push(add_message)
+      ::Gitlab::Audit::EventQueue.push(remove_message)
     end
   end