Update Operator installation instructions for OCP 4

The existing operator installation documentation will need to be extended to cover operator installation on OCP 4, in addition to the existing operator installation on OCP 3. There are bound to be some differences in the details of how this is done.

This will affect Ch. 6 "Installing the 3scale Operator" and section 2.7 "Deploying 3scale using the operator" from the Installing 3scale guide [1].

[1] https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.5/html/installing_3scale/installing-3scale-operator