Remove 3scale operator for capabilities mention as a Technology Preview feature only

In Chapter 8. Using the 3scale operator to configure and provision 3scale there is a warning informing that the 3scale operator for capabilities is a Technology Preview feature only. From 2.14, this feature is supported, so the warning can be adapted with the disclaimer included in the previous paragraph:

While new services and resources appear in the Admin Portal, you cannot update the service or resource in the Admin Portal or by using the internal 3scale API. If you try to, the operator reverts updates and the CR remains as it was.