Request to prepend_mod for pages available

What does this MR do and why?

Request to prepend_mod for pages available

Because the upstream MR removed the project#pages_available? method: !133216 (merged)

But JH has own business logic in it:

https://jihulab.com/gitlab-cn/gitlab/-/blob/main-jh/jh/app/models/jh/project.rb#L79

So we need to keep it in lib/sidebars/projects/menus/deployments_menu.rb.