Refactor for_gitlab_duo_pro and user.duo_pro_add_on_available? usage

What does this MR do and why?

We should refactor the code to utilize CloudConnector::AvailableServices, for both Gitlab.com and Self-managed instances as part of our effort to decouple how the add-ons are bundled with the features.

We introduced a method in CloudConnector::BaseAvailableServiceData in !152407 (merged)

    def purchased?
      add_on_purchases.any?
    end

and this MR replaces any occurrences of ::GitlabSubscriptions::AddOnPurchase.for_gitlab_duo_pro.any?

- ::GitlabSubscriptions::AddOnPurchase.for_gitlab_duo_pro.any?
+ ::CloudConnector::AvailableServices.find_by_name(:dou_chat or :code_suggesitons).purchased?

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